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Electrochemical brightening of pulp with sodium dithionite generated in-situ Hu, Hong-Liang 1994

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ELECTROCITEMICAL BRIGHTENING OF PULP WITH SODIUM DIfILLON1TE GENERATED IN-SITU by Hong Liang Ru -  B. Eng., East China University of Chemical Technology, 1990  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in TIlE FACULTY OF GRADUATE STUDIES DEPARTMENT OF CHEMICAL ENGINEERING We accept this thesis as conforming to the required standard  THE UNIVERSITY OF BRITISH COLUMBIA February, 1994 © Hong Liang Ru, 1994 -  In presenting this thesis in  partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my permission.  (Signature)  Department of The University of British Columbia Vancouver, Canada Date  DE-6 (2/88)  written  Abstract  This study consisted of three parts, an investigation of potential reducing agents, an investigation of conventional dithionite brightening at 1% pulp consistency and an investigation ofin-situ electrochemically generated dithiomte brightening.  Preliminary brightening experiments by four reducing agents, hydrazine sulfate, hydroquinone, hydroxylamine sulfate and sodium hypophosphite were conducted. At 600C, with 1 hour brightening period, 4 to 16 wt% brightening agent charge and 0.5 wt% chelating agent charge on od pulp under nitrogen purge, these agents could only achieve a  brightness gain of up to 4 % ISO. They were less effective than sodium dithionite in mechanical pulp brightening.  Sodium dithionite brightening of softwood TMP at 600C and 1% pulp consistency was done with a single addition of dithionite, multiple addition of dithionite and addition of dithionite in multiple stages with interstage washing. With a single addition of the same dithionite charge, the brightness gain at 1 % pulp consistency was equal to that in conventional brightening at 3 to 4% pulp consistency. For a given total sodium dithionite charge on od pulp, addition of clithionite in multiple stages with interstage washing could achieve about 1 % ISO higher brightness gain than multiple addition of dithionite in a single run, while multiple addition of dithionite in a single run could achieve about 1 % ISO higher brightness gain than a single addition of dithionite. The time and amount of each charge did not seem to affect the brightness gain for multiple additions of dithiomte.  In dithionite brightening with a large amount of(bi)sufflte (13 to 131 wt% sodium  sulfite on od pulp), the brightness gain was about 3 % ISO lower than that with only dithionite, but about 4 % ISO higher than that with only sulfite. Sodium bisulfite and II  dithionite reaction products seemed to decrease dithionite brightening capability. The addition of 2-propanol or methanol could alleviate this negative effect of sodium bisuffite on dithionite brightening by about 1% Iso.  The electrochemical synthesis of sodium dithionite (without pulp) was conducted in a 1 L mixed tank batch reactor evenly divided by a cation exchange membrane with . Runs were carried out at 40 °C and 60 °C. High 2 cathode area of 5500 to 13000 mm time average dithionite concentration (5 to 10 g/l) and fairly good current efficiency (35 80  %) were obtained at high current ( 2.3  -  A) together with high sodium sulfite dose (  25.3 g). Dithionite concentration became very low ( 0 to 1 g/l) at low sulfite dose (4.6 g). Cathode area, current and sodium sulfite dose were very significant variables in this process. Increases in these variables raised the time average dithionite concentration. Increases in cathode area as well as increases in sulfite dose raised the final current efficiency for dithionite generation. Decreases in current raised final current efficiency with the cathode of 5500  2 nl.m  area, but did not have significant effect with cathode of 13000  area.  In-situ electrochemically generated sodium dithionite brightening at ambient temperature was briefly investigated in Plexiglas reactor of the same dimensions as the 2 area. These runs gave a maximum reactor given above, but with a cathode of 1000 mm brightness gain of about 5 % ISO. In-situ electrochemically generated sodium dithionite brightening at elevated temperature was extensively investigated with the  same  reactor and  cathodes used in the electrosynthesis of dithionite. When electrochemical dithionite brightening was carried out at 0.8 % consistency in the following ranges: pH (4.0 to 5.5), sodium sulfite dose (4.6 to 46.0 g), current (0.5 to 4.0 A) and temperature (40 to 60 oC), the highest range of brightness gain was 8 to 9 % ISO with time average dithionite concentration equivalent to a charge of 10 to 103 wt% on od pulp. This highest range of 111  brightness gain was obtained at any pH with a combination of high sulfite dose (46.0 g), high temperature (60 °C) and high current (4 A). At pH 5.5, this highest range of brightness gain was also obtained with a combination of high sulfite dose (46.0 g), high temperature (60 °C) and low current (0.5 A). Temperature had the most significant effect on brightness gain in this process, increases in temperature raised brightness gain but lowered the final net current efficiency for dithionite generation. Increases in cathode area, addition of 2-propanol and chromium did not raise this highest range of brightness gain in the ranges of operating variable studied.  In the subsequent optimization of brightness gain for in-situ electrochemically generated dithionite brightening at 2% consistency, the highest range of brightness gain obtained was 11 to 12 % ISO, which was only obtained at the highest range of temperature (74 to 83  oC),  with time average dithionite concentration equivalent to a  charge of 7 to 10 wt% on od pulp along with a very poor final current efficiency for dithionite (0 to 13 %). Yellowness decreased in electrochemical dithionite brightening in the same way as in conventional dithionite brightening.  Comparing the brightness gain of the blank experiment (no current) to the highest brightness gain obtained from the electrochemical dithionite brightening at the same sodium suffite dose, temperature, pH and pulp consistency, the maximum further brightness gain contributed by the brightening species generated by current was only about 4 % ISO. For a brightness gain of 11.4 % ISO, the fraction of the brightness gain contributed by the brightening species generated by current was about 26 % of the total brightness gain.  iv  TABLE OF CONTENTS Page Abstract  ii  Table of Contents  v  List of Tables  vii  List of Figures  x  Nomenclature  xii  Acknowledgment  xiv  1 Introduction  1  2 Literature Review  2  2.1  Wood Chemistry  2  2.2  Chemical Principles of Brightening  5  2.3  Brightness  8  2.4  Brightening Chemicals  9  2.5  Sodium Bisuffite Brightening  11  2.6  Sodium Dithionite Brightening  15  2.7  In-Situ Electrochemical Bleaching and Brightening of Pulp  21  2.8  Electrochemical Synthesis of Sodium Dithionite  22  2.9  Factorial Design and Analysis of Experiments  27  2.10  Sequential Simplex Optimization  29  3 Experimental Apparatus and Methods  30  Experimental Procedure and Apparatus for Sodium Dithionite Analysis  31  3.2  Conventional Brightening of Mechanical Pulp  34  3.3  Electrochemical Generation of Sodium Dithionite  37  3.4  Brightening Mechanical Pulp by In-Situ Generation of Sodium Dithionite  37  3.1  V  3.5  Software  43  4 Experimental Results and Discussion 4.1  45  Preliminary Investigation on Mixing Quality versus Pulp Consistency in Laboratory Reactor  45  4.2  Preliminary Investigation of Possible Brightening Agents  47  4.3  Sodium Dithionite Brightening at 1% Pulp Consistency  51  4.4  Electrochemical Generation of Sodium Dithionite at Elevated Temperature  58  In-Situ Electrochemically Generated Dithionite Brightening at Ambient Temperature  62  In-Situ Electrochemically Generated Dithionite Brightening at Elevated Temperature  66  Optimization of Brightness Gain in In-Situ Electrochemically Generated Dithionite Brightening at 2% Consistency  76  Blank Experiments without Current  78  4.5  4.6  4.7  4.8  5 Conclusions and Recommendations  79  5.1  Conclusions  79  5.2  Recommendations  82  Bibliography  83  A Measurement of Pulp Stock Consistency  87  B Pulp Specification 88 C Real Surface Area of Cathode Screen  89  D Experimental Results  90  E Sample Calculations  111  Glossary  112  v  List of Tables Page 2.1  Average composition of softwood and hardwood  2.2  Commercial brightening processes  10  2.3  Cost effective brightening processes (softwood)  10  2.4  Preferred dithionite brightening process conditions  16  2.5  Major reactions in electrosynthesis of sodium dithionite  23  2.6  Two variable factorial design  27  3.1  Cathode superficial areas immersed in pulp suspension  38  4.1  Mixing quality versus pulp consistency in simulated laboratory reactor  46  4.2  Reductive brightening agents in trial  47  4.3  Hydrazine sulfate brightening  48  4.4  Hydroquinone brightening  49  4.5  Hydroxylamine sulfate brightening  49  4.6  Sodium hypophosphite brightening  50  4.7  Runs with high brightness gain obtained with two different cathode configurations at high level of sodium suffite dose  69  3  D. 1 Brightness responses versus charge at 1% consistency  90  D.2 Multiple charge dithionite brightening at ambient temperature  91  D.3 Multiple charge dithionite brightening at elevated temperature  91  D.4 Multiple charge dithionite brightening with sodium sulfite at elevated temperature  92  D.5 Multiple charge dithionite brightening with sodium suffite and suppressors at elevated temperature  93  D.6 Operating conditions of dithionite synthesis at elevated temperature with rectangular cathode  94  D.7 Symbols used in factorial design of dithionite synthesis at elevated temperature with rectangular cathode  94  vil  D.8 Experimental variables and results of dithionite synthesis at elevated temperature and with rectangular cathode  95  D.9 Comparison of the effect 4 SO on generating sodium dithionite 2 ofNa  96  D.10 Factorial analysis of dithionite synthesis at elevated temperature with rectangular cathode  96  D. 11 Operating conditions of dithionite synthesis at elevated temperature with half cylinder cathode  97  D.12 Symbols used in factorial design of dithionite synthesis at elevated temperature with half cylinder cathode  97  D. 13 Experimental variables and results of dithionite synthesis at elevated temperature with half cylinder cathode  98  D.14 Repeated runs of dithionite synthesis at elevated temperature with half cylinder cathode  99  D. 15 Factorial analysis of dithionite synthesis at elevated temperature with half cylinder cathode  99  D. 16 Operating variables and experimental results of in-situ electrochemically generated dithionite brightening at ambient temperature  100  D. 17 Operating conditions in electrochemical dithionite brightening with half cylinder cathode at low sodium sulfite dose  101  D. 18 Symbols used in factorial design of electrochemical dithionite brightening with half cylinder at low sodium sulfite dose  101  D. 19 Experimental variables and results in electrochemical dithionite brightening with half cylinder cathode at low sodium sulfite dose  102  D.20 Factorial analysis of electrochemical dithionite brightening with half cylinder cathode at low sodium sulfite dose  103  D.21 Operating conditions of electrochemical dithionite brightening with half cylinder cathode at low sodium suffite dose  103  D.22 Symbols used in factorial design of electrochemical dithionite brightening with half cylinder cathode at high sodium suffite dose  vrn  103  D.23 Experimental variables and results of electrochemical dithionite brightening with half cylinder cathode at high sodium sulfite dose  104  D.24 Factorial analysis of electrochemical dithionite brightening with half cylinder cathode at high sodium sulfite dose  105  D.25 Experimental variables and results of electrochemical dithionite brightening with rectangular cathode at high sodium sulfite dose  105  D.26 Operating conditions in electrochemical dithionite brightening with 2-propanol  106  D.27 Experimental variables and results of electrochemical dithionite brightening with 2-propanol  107  D.28 Operating conditions in electrochemical dithionite brightening with chromium  108  D.29 Experimental variables and results of electrochemical dithionite brightening with chromium  108  D.30 Experimental conditions of sequential simplex optimization  109  D.31 Experimental variables and results of sequential simplex optimization  109  D.32 Operating conditions in blank experiments  110  D.33 Experimental results of blank experiments  110  ix  List of Figures Page 2.1  Cellulose structure  3  2.2  A structure segment of softwood lignin  4  2.3  Basic chromophore structures in wood  6  2.4  Sodium dithionite brightening mechanism  7  2.5  Effect of chemical charge on NaHSO 3 brightening  12  2.6  Effect of reaction time on NaHSO 3 brightening  12  2.7  Effect of temperature on NaHSO 3 brightening  13  2.8  Effect of consistency onNaHSO 3 brightening  13  2.9  3 brightening Effect of pH on NaHSO  14  2.10 Effect of charge on brightness gain in dithionite brightening  17  2.11 Effect of pH and temperature on brightness gain in dithionite brightening  19  3.1  Apparatus for dithionite titration  33  3.2  Apparatus for conventional brightening  35  3.3  Apparatus for brightening mechanical pulp by in-situ generating sodium dithionite  41  Brightening responses of dithionite brightening versus charges at 1% pulp consistency  52  4.2  Multiple charge dithionite brightening at ambient temperature  53  4.3  Multiple charge dithionite brightening at elevated temperature  55  4.4  Multiple charge dithionite brightening with sodium sulfite at elevated temperature  56  Multiple charge dithionite brightening with sodium suffite and suppressors at elevated temperature  57  Dithionite concentration and pH profile in electrochemical dithionite brightening at ambient temperature (Run Cia)  63  4.1  4.5  4.6  x  4.7  4.8 4.9  Dithionite concentration and pH profile in electrochemical dithionite brightening at ambient temperature (Run C3)  64  Brightness responses of electrochemically generated dithionite brightening versus brightening time  65  Sequential simplex optimization of electrochemical brightening  77  xi  Nomenclature  A  . 2 real surface area, mm  0 A  . 2 real surface area of a plate, mm  1 A  2 real surface area of screen 1, mm  2 A  . 2 real surface area of screen 2, mm  A  . 2 cathode surface area, m  C  sodium suffite dose, g.  C  final concentration of dithionite, mole/i.  CI  confidence intervals.  D  nominal wire diameter, mm.  F  Faraday constant.  i  . 2 current density, Amp/rn  I  current, Amp.  k  number of separate estimates.  1 k  rate constant of dithionite formation reaction, m  2 k  rate constant of dithionite decomposition reaction,  L  length.  M  screen mesh number.  n  electron stoichiometry coefficient.  N  number of factorial runs in the design.  rj  number of replicates.  R  average reaction rate, mole/sec.  R’  universal gas constant, 8.31 kJ/kmole/K.  s  response error.  sj  response error.  t  reaction time, sec. xli  . 1 r  t’  Student’s t statistics.  T  temperature, 0 C.  U  duplicated response.  v  degrees of freedom.  V  reaction volume, liter.  V°  standard electrode potential, Volt.  yr  reversible electrode potential, Volt.  W  duplicated response.  Xl  variable.  X2  variable.  Yl  response.  Y2  response.  Y3  response.  Y4  response.  xlii  Acknowledgment  My sincere thanks to Professor Cohn Oloman for his advice and help in this work.  Also acknowledged are the staff of Department of Chemical Engineering and Pulp and Paper Center at University ofBritish Columbia for their help.  I would like to thank all of our group members for their help, particularly Jenny Been, Rajagopal Srinivasan and Elod Gyenge.  Finally, I would like to thank the Mechanical and Chemimechanical Wood-puips Network for its financial support.  xiv  Chapter 1. Introduction Chapter 1  Introduction  The main problems facing mechanical pulp brightening are the needs to achieve as high a brightness as that of bleached chemical pulp and to eliminate brightness reversion. In mechanical pulp brightening, hydrogen peroxide and sodium dithionite are the commercial brightening agents. Usually if a brightness gain of 6  —  8 % ISO is needed,  dithionite is employed. If a brightness gain of 10— 18 % ISO or even higher is needed, then peroxide or peroxide  —  dithionite two stage brightening is employed [Andrew 1982].  There is a strong demand for economic, environmentally friendly brightening agents capable of achieving about 20 % ISO brightness gain with mechanical puips. Thus it is the objective of this study to investigate such new brightening agents or processes. It is known that sodium dithionite is unstable and quickly consumed or decomposed after its addition into a pulp suspension. A consistently high level of dithionite will be present in the pulp suspension if an eleetrochemical method is utilized to generate sodium dithionite from sodium bisuffite inside the brightening reactor. Also the in-situ generation of dithionite may produce some transient chemical species which enhance the brightening process. In-situ generation, along with a continuously high dithionite concentration, might yield about 20 % ISO brightness gain. Thus it is the main objective of this study to investigate the possibility of brightening by in situ electrochemically generated dithionite to achieve a high brightness gain. Up till now, no one has ever studied such a process. No information on reactor design or process conditions is available. Thus this is a highly speculative research project.  1  Chapter 2. Literature Review Chapter 2  Literature Review  2.1 Wood Chemistry  The main chemical components of wood are cellulose, hemicellulose, lignin and extractives. Cellulose is a long chain, unbranched polymer of glucose, the degree of polymerization (or DP) is in the range of 600  —  1500. It is also known as alpha cellulose.  The structure is shown in Figure 2.1. Hemicellulose is a short chain, branched polymer of five different sugars: glucose, mannose, galactose, xylose and arabinose. Hemicellulose can be categorized according to DP: (a). beta cellulose (b). gamma cellulose  —  —  DP in the range of 15 to 90,  DP less than 15. Both cellulose and hemicellulose are  polysaccharides. Lignin is a highly branched, three-dimensional polymer network consisting of aromatic rather than saccharide building units. Its basic unit is phenyl propane. The structure of lignin is shown in Figure 2.2. Lignin bonds the cellulose fibers  and other components of the wood together. Extractives are those miscellaneous, small quantity low molecular weight inorganic and organic substances present in wood, such organic substances are, e.g. resin acids, fatty acids, turpenoid compounds and alcohols. Most of these extractives are soluble in water or neutral organic solvents. The average compositions of softwoods and hardwoods are listed in Table 2.1.  Pulp is the fibrous material made from wood or other cellulosic raw materials and used for papermaking. The process of making pulp is called pulping. Pulping is achieved by chemical means, mechanical means or combinations of the two, such as chemi mechanical pulping and semi-chemical pulping. In mechanical pulping, most of the defibration energy comes from mechanical energy (mechanical force and steam), so 2  Chapter 2. Literature Review chemical constituents of the fibrous materials are essentially retained, except for extractives. Pulp yield is very high in this process, usually around 90  —  95% [Smook  1992, 44]. Pulp made from this process is called mechanical pulp. In chemical pulping, most of the defibration energy comes from chemical energy, most of the fiber bonding substance, lignin, is chemically dissolved so that fibers are separated from the woody matrix, and some of the hemicellulose and cellulose is also dissolved. Pulp yield; in chemical pulping is usually low, about 40—50% [Smook 1992, 44]. Pulp made from this process is called chemical pulp.  Table 2.1. Average composition of softwood and hardwood [Smook 1992, 151 Softwoods  Hardwoods  Cellulose  42±2%  45±2%  Hemicellulose  27±2%  30±5%  Lignin  28±3%  20±4%  Extractives  3±2%  5±3%  Secondary hydroxyl_  _Primary hydroxy I H CH O 2  HOH H  H  HO H 0  H  Non-reducing end group  o.  H  I  a  o  I OH  cI-IOH H  j;fD H  H  H  H CH O 2  OH  H CH O 2  H  OH  H  Cellobiose units  Figure 2.1. Cellulose structure [Smook 1992, 5]  3  H  i4zx  n  L.  a  OH  Reducing  end group  Chapter 2. Literature Review  ?hb0H  Figure 2.2. A structure segment of softwood lignin [Alder 1977)  4  Chapter 2. Literature Review 2.2 Chemical Principle of Brihtenin2  The color of unbleached chemical pulp or unbrightened mechanical pulp is yellow or brown. From the wood chemical component point of view, polysaccarides  —  cellulose  and hemicellulose without contamination are white. Lignin and most organic extractives show color, and their color even intensifies after mechanical pulping because new or darker chromophores form during the pulping process. Bleaching is only applied to chemical pulp and semi-chemical pulp, while brightening is only applied to mechanical pulp and chemi-mechanical pulp. The main purpose of bleaching and brightening is the same  —  to increase the brightness of pulp. Bleaching substantially dissolves the residual  color-producing lignin and leaves a relatively white, pure cellulosic fiber, while brightening selectively converts color-producing chromophores to relatively colorless entities without appreciable dissolving major components of wood, so that the main advantage of mechanical pulping and chemi-mechanical pulping, i.e. high yield, can be preserved. However the brightness limit of brightening, which is around 82 % ISO and poor brightness stability still limit the extensive use of mechanical pulp and chemi-mechanical pulp.  Carbonyl groups and double bonds are the main chromophore groups in lignin and related components of wood. When they are combined with benzene rings and auxochromes in a suitable way, they form chromophoric structures which move the wavelength of light absorption from the ultraviolet region into the visible region, and generate the yellow-brown color of wood [Polcin and Rapson 19711.  Three major types of basic chromophoric structures [Fleury and Rapson 1968] in wood are: (a). ortho- and para- quinones (Ia, Ib) 5  Chapter 2. Literature Review (b). ortho-hydroxyl- and para-hydroxyl-phenyl ketones (Ha, llb) (c). para-quinone methides and carbonium ions (ifia, ifib) These structures are shown in Figure 2.3.  Ici  lb°  Figure 2.3. Basic chromophore structures in wood [Fleury and Rapson 19681  The general principle of brightening is to break the conjugation between individual chromophores, or change the chemical structure of chromophores, or eliminate auxochromes, or break the chemical bonds between auxochromes and chromophores. However, the detailed mechanism of brightening is still unknown.  6  Chapter 2. Literature Review Individual reducing brightening agents have different reactivity towards different types of carbonyls. Dithionite primarily reduces simple quinone structures to hydroquinone structures and alpha, beta-unsaturated aldehyde and ketone structures to alcohol structures. These mechanisms are shown in Figure 2.4. However, some of the reduced alcohol structures formed during dithionite brightening are easily oxidized back to their  original structures, leading to brightness reversion [Joyce and Mackie 1979]. Borohydride and uranium(ffl) can, in addition, also eliminate ring conjugated carbonyls effectively.  OH  Reduces quinone structure to hydroquinone structure  Reduces alpha/beta unsaturated aldehyde and ketone structure to  H R.  _O  II H-C  H—C—OH U H-C  alcohol structure  Figure 2.4. Sodium dithionite brightening mechanism [Bennington 1991]  7  Chapter 2. Literature Review 2.3 Brightness  Brightness and yellowness are two important optical properties of pulp. Brightness, more precisely papermaker’s brightness, is defined as the reflectance of blue light with a specified spectral distribution peak at 457 nm from an opaque surface of pulp sheets compared to a specified reflecting, diffusion standard surface [Smook 1992; CPPA 1990]. Visual brightness is subjective and there is no general agreement between papermaker’s brightness and visual brightness. Two widely accepted methods of measuring brightness are that of General Electric (Tappi Standard T452, U.S.A.) and the Zeiss Elrepho (official standard all over the world, except the U.S.A.). In this study, the CPPA method (Zeiss Elrepho) is used. This method specifies that the sample should be diffusely illuminated with a highly reflecting, integrating sphere. Reflected light is measured 900 to the sample, and reflectance is compared to absolute reflectance from an imaginary perfectly reflecting, diffusing surface, and the ratio of the reflectance to absolute reflectance is taken as brightness. The standard is opal glass. The brightness thus obtained is the “ISO brightness” expressed as a % ISO. For general commercial uses, MgO powder is used as a brightness standard, which is about 98-99% of absolute reflectance. In this case, the brightness is Elrepho brightness or MgO brightness. There is no relationship between the GE and the ISO brightness scale, but generally GE brightness is about 1 % lower than ISO brightness.  Yellowness is the difference between luminance and brightness values expressed as a percentage of reflectivity. It is a measure of how yellow or off white a paper looks [Smook 1992].  8  Chapter 2. Literature Review 2.4 Bri2htenin2 Chemicals  Commercial brightening chemicals are sodium bisulfite (or other sulfur dioxide derivatives), sodium dithionite and hydrogen peroxide. Among them, sodium bisulfite and sodium dithionite are reducing brightening agents, while peroxide is an oxidizing agent. Sodium borohydride is not a commercial brightening chemical, but only used for on-site generation of sodium dithionite from SO , as in the Borol Process. Table 2.2 lists some 2 common brightening practices and their typical brightness gains. The most cost effective brightening processes for softwood are summarized in Table 2.3. The chemical cost for peroxide  —  dithionite brightening in Table 2.3 is about 40 $/ton od pulp [Joachimides  1989].  9  Chapter 2. Literature Review Table 2.2. Commercial brightening processes [Dessureault 1991; Joachimides 1989] Brightening Chemical Process Maximum Brightness Gain (Spruce/Balsam) % ISO 6 Chips Sodium Sulfite 4 Interstage 3 Refiner 3 Latency chest Low consistency in tower 8 Dithionite (3-4%) High consistency in tower 10 (10-12%) 11 Refiner only + 14 Refiner post refiner 18 Chips Peroxide Interstage Rejects Post refiner: Single stage, M.C. or H.C. Two stages, M.C. + M.C. Two stages, M.C. + H.C. 20 Oxidation Reduction Post refiner: + dithionite Peroxide Peroxide + dithionite + peroxide Dithionite + peroxide + peroxide -  Table 2.3. Cost effective brightening processes (softwood) [Joachimides 1989] Most Cost Effective Process  Brightness Goal  Brightness of 65 to 70 % ISO or Refiner dithionite brightening for refiner based mills  brightness gain of 12 to 14 % ISO  Brightness of 70 to 80 % ISO or Peroxide  —  dithionite brightening  brightness gain of 15 to 18 % ISO Brightness of more than 80 % ISO or Peroxide brightness gain of more than 20 % ISO  10  —  peroxide brightening  Chapter 2. Literature Review 2.5 Sodium Bisulfite Brhhtenin  Sodium bisulfite has a relatively low brightening effect because it can only reduce chromophores of quinone and quinone methide structures in wood. These structures are reduced to phenol structures which are sometimes difficult to oxidize. Consequently, bisulfite brightened pulp has a high brightness stability. Hence, it is normally used to achieve small brightness gain, less than 6 % ISO brightness. For this purpose, it has advantages over dithionite in that it is more cost effective and its process is much simpler. It does not require a pretreatment, stabilization and pH adjustment stage and there is no corrosion problem at all since sodium thiosulfate does not exist in the process. The ideal place to apply sodium bisulfite is in the storage chest in which pulp is often stored for several hours at high temperature. For example, pulp of 10% consistency stored at 83oC for 24 hours will lose 5 % ISO brightness. By applying sodium bisulfite, brightness increases, rather than decreases.  The main variables of bisuffite brightening are chemical charge, reaction time, pH, temperature and pulp consistency. Their effects on brightening are discussed in the following paragraphs [Kutney and Evans 1986]:  At 700C with 20% pulp consistency and 2 hour brightening period, a small amount of bisuffite, e.g. 1.5 wt% on od pulp can give a brightness gain of 3 to 4 % ISO. Increases in brightness gain can be achieved with increases in bisulfite charge, reaction time, reaction temperature and pulp consistency. However, pH has little effect on brightness gain over the range of 3 to 10. The effects of these variables on brightness gain are shown in Figure 2.5 —2.9. Units of the brightness gain in the following figures are % ISO.  11  Chapter 2. Literature Review Generally, with 1.6 wt% sodium bisuffite on od pulp, 2 to 4 % ISO brightness gain  can be achieved at low consistency and 4 to 6 % ISO brightness gain at medium consistency.  z I-) (I) U)  w  z m  CHEHICAL CHARGE C Z ON PULP )  3 brightening (upper Figure 2.5. Effect of chemical charge on NaHSO 70°C! 3 3 ) Conditions: NaHSO line: Na ; lower line: NaHSO 4 O S 2 DTPA pretreatedfN 2 4 O S 2 2 his /20% Consistency; Na purge/70°C/ 40 mm. /pH 6 (NaOH) /4% Consistency [Kutney and Evans 1986] —  —  z CD U) (I) Li  z  I— ID 03  TIME ( hr. )  3 brightening. Figure 2.6. Effect of reaction time on NaHSO 3 on od pulp Conditions: 60°C14% consistency/ pH 6/3.5 wt°h NaHSO [Kutney and Evans 19861  12  Chapter 2. Literature Review  I  I .  TEMP.  85  (C)  3 brightening. Figure 2.7. Effect of temperature on NaHSO 3 on od pulp Conditions: 1 hr/4% consistency! pH 6/3.5 wt% NaHSO [Kutney and Evans 1986]  U0  I  5  10 15 CONSISTENCY  20  3 brightening. Figure 2.8. Effect of consistency on NaHSO 3 on od pulp [Kutney Conditions: 60°C/i hr/ pH 6/ 3.5 wt% NaHSO and Evans 1986]  13  Chapter 2. Literature Review  6  0  A 0  U) U)  0  0 0  0  w z I  ci I-I  w n “3  I  I  I  I  I  4  5  6  7  8  9  10  3 brightening. Figure 2.9. Effect of pH on NaHSO 3 on cxl pulp Conditions: 60°C/4% consistency! I hr/ 3.5 wt% NaHSO 1986] [Kutney and Evans  14  Chapter 2. Literature Review 2.6 Sodium Dithionite Brightening  A recent survey of the Canadian pulp and paper industry reveals that the single stage dithionite brightening process is the dominant mechanical pulp brightening process because this is the most economic process for limited brightness improvement [Mcdonough 1992]. The brightness of unbrightened mechanical puips and their brightening response depend on wood species, storage conditions, general condition of wood, age of chips and mechanical pulping process [Goel 1982; Leask et al. 1987]. Brightening by dithionite or peroxide can not eliminate the brightness difference caused by the above mentioned factors. The preferred dithionite brightening process conditions are listed in Table 2.4, and the main reactions of sodium dithionite during brightening are listed below [Kerekes et al. 1991].  Reactions of sodium dithionite during brightening 1. Brightening 4 +1120 O S 2 Na  +  Lignin  -  3 2NaHSO  +  Brightened lignin  2. Disproportionation in absence of air 4 O S 2 2Na  +  0 2 H  —*  3 2NaHSO  +  3 O S 2 Na  3. Oxidation with excess air (entrained in pulp) 4 +02+1120 O S 2 Na  -*  3 NaHSO  +  4. Oxidation with limited air 4 +02+ 21120 O S 2 Na  —  3 4NaHSO  15  4 NaHSO  Chapter 2. Literature Review Table 2.4. Preferred dithionite brightening process conditions Low Consistency  Medium Consistency  Pulp Consistency  3 —4%  8  Temperature  60°C  75  Time  1 —2 hours  20 minutes  pH  —  12%  —  85°C  5—6  Charge ofNa O4 S 2  0.5  Chelating Agents  0.2  —  —  1.0 wt% on od pulp 0.5 wt% on od pulp  Exclusion of air  These conditions are chosen to reach required brightness with minimum cost. In sodium dithionite brightening, careful control of brightening conditions is very important, especially pH and air. Brightening performance is primarily affected by dithionite charge, pulp consistency, exclusion of air, pH, temperature and reaction time as well as chelating agents. These process variables are discussed separately as following:  Dithionite Charge  When other variables are kept at their preferred conditions, the brightness increases with the increase in dithionite charge, until about 1.0  —  1.5% sodium dithionite  on od pulp. Beyond this charge level, brightness levels off. Charge effect on brightness gain is illustrated in Figure 2.10 for typical brightening conditions. The shape of the brightness response indicates a brightness gain limit. For mechanical pulp, this limit is about8—10%ISO.  16  Chapter 2. Literature Review  I  C C  0.25  0.50  4 0 S 2 Conc’nNa  075  (o  .WO 0.0.  1.25  1.50  pulp)  Figure 2.10. Effect of charge on brightness gain in dithionite brightening [Andrews 1982] (. includes STPP, I dithionite only)  Pulp Consistency [Andrews 1982; Singh 1979]  The magnitude of this variable is a compromise between process economy and dithionite instability. Too low or too high consistency results in a loss of dithionite efficiency. At too low consistency, water contains sufficient oxygen to partly destroy dithionite. Also too low consistency is not desirable in terms of process economy. While at too high consistency, the pulp suspension tends to entrain air, whiöh can destroy much of the dithionite before it brightens pulp. It is also difficult to achieve good mixing of the active dithionite and pulp at high consistency. However, effective brightening at high consistency is possible provided air is eliminated [Singh 1979]. Thus for single stage dithionite brightening, low consistency at 3 consistency at 8  —  —  4% is the common practice. Medium  12% is sometimes used in the second stage of peroxide  two stage brightening.  17  -  dithionite  Chapter 2. Literature Review  nil The pH effect on brightness is shown in Figure 2.11 [Leask et al. 1987, 232]. This effect is, to some extent, interdependent on the temperature effect. In general, brightness gain decreases rapidly above pH 7, and more gradually below the optimum range pH 5— 6. Below pH 4, sodium dithionite decomposes rapidly. Although sodium dithionite is more stable and its thermodynamic reducing power is higher under alkaline condition than under acidic condition, dithionite brightening can not be carried out under alkaline condition, perhaps because the active brightening agent is some intermediate species formed at lower  pH, or probably because the brightening reaction is too slow at pH above 7. In addition alkali darkening becomes important at pH above about 10.  Time and Temperature [Leask et al. 1987; Singh 1979; Andrews 19821  Figure 2.11 shows the effects of pH and temperature on brightening for typical dithionite brightening conditions. Generally, brightness gain increases with temperature until a certain limit (80°C), as illustrated in Figure 2.11. However, brightening at too low temperature, i.e. 0°C by any amount of dithionite will not achieve any brightness gain. Although higher brightness gain can initially be achieved at higher temperature, brightness loss after brightening also increases with higher brightening temperature. Brightening at  higher temperature (80°C) should be accomplished in a shorter time (15 mm.) in order to avoid greater brightness loss after brightening. At a given dithionite charge, brightness develops faster at higher temperature. Brightness increase is rapid initially, but after a few minutes the rate of increase diminishes. Brightness increase may last several hours, and finally levels off. For example, at 60°C, 3.3% pulp consistency and 0.9 wt% charge on od pulp, 75% of the maximum brightness is achieved in the first 5 mm. and 90% in 20 mm. However, at 40°C, 75% of the maximum brightness is achieved in the first 20 mm. and 18  Chapter 2. Literature Review 90% in 60 mm. At a lower level of dithionite charge, it takes a shorter time to achieve maximum brightness.  67 66 65 C,, C,)  w z I  63  cc °  62 pH  Figure 2.11. Effect of pH and Temperature on brightness gain in dithionite brightening [Leask 1982] (Brightness units are % ISO)  Chelating Agents [Joyce 1979]  Some heavy meta1, particularly iron and manganese, reduce brightness gain and increase brightness reversion, and their detrimental effects can be suppressed by the use of chelating agents, such as STPP (sodium tripolyphosphate),  TSSP (tetrasodium  pyrophosphate) or the more costly sodium salts of EDTA (ethylenediamine tetraacetic acid) or DTPA (diethylene triamine pentaacetic acid). If ferric ions exist in pulp, dithionite will reduce it to ferrous ion. Ferrous ions will be oxidized back by oxygen in air and form a highly colored ferric phenolic complex. STPP is widely used in dithionite brightening, 19  Chapter 2. Literature Review and sometimes can increase brightness gain by 2 agents, such as 0.1  —  —  4 % ISO. Addition of chelating  0.5 wt% STPP and DTPA on od pulp, before brightening produces  higher brightness, by 0.5  —  1.0 % ISO, than addition along with dithionite.  20  Chapter 2. Literature Review 2.7 In-Situ Electrochemical BleachinE and Brihtenin of Pulp  There are few papers about in situ electrochemical bleaching and brightening pulp. Bleaching of chemical pulp by electrolysis of sodium chloride solution containing pulp suspension has been investigated [Nassar 1985; Wilk 19891. In this process, the main reactions in the anode chamber are chlorine and hypochiorite generation. Both are conventional bleaching chemicals. Brightness of 70 % ISO for kraft pulp and 75 % ISO for suffite pulp are achieved with acceptable viscosity [Nassar 1985]. With BrinceIl’Tht electrolysis cell, ozone is also generated and its concentration is around 10 mg/i. This ozone reduces the color level of the effluent [Wilk 1989]. Electrochemically mediated oxygen delignification of kraft pulp has also been extensively studied at TJBC [Perng 1993; Todd 1993]. However, the mediator (redox couple) enhances the delignification and degradation of cellulose at the same time.  In-situ reductive brightening of mechanical pulp with cathodically generated EDTA-dipositive chromium complex has also been studied [Hull and Yasnovsky 1986]. In this process, EDTA-Cr(Ifl) initially added is reduced to EDTA-Cr(ll) at the cathode, and the EDTA-Cr(ll) brightens mechanical pulp and itself is oxidized back to EDTA-Cr(ffl). Thus the brightening agent EDTA-Cr(ll) cycles. Brightness gain of 21 % ISO or reversed brightness gain of 9 % ISO is achieved. Brightness gain is raised by decreasing pH (3 <pH< 6), increasing temperature (30°C <T< 90°C), reaction time (1 hour <t< 6 hours), and initial concentration of EDTA-Cr(llI) (1 mM/I  <  [EDTA-Cr(ffl)]  <  However, brightness gain changes little with consistency in the range of 1 to 3%.  21  5 mMJ1).  Chapter 2. Literature Review 2.8 Electrochemical Synthesis of Sodium Dithionite  Sodium dithionite can be prepared electrochemically by cathodic reduction of sodium bisulfite or sulfur dioxide solution [Oloman and Austin 1969; Oloman 1970; Oloman et al. 1990; Leutner, et al. 1979; Bolick, II, et al. 1988; Cawifield, et al. 19881. Currently there are two commercial plants making sodium dithionite using electrochemical technology, this electrochemical process is more cost-competitive than the conventional thermochemical processes, such as the borohydride process in pulp and paper industiy. Sodium dithionite from this electrochemical process is widely used in bleaching minerals  and brightening pulp [Kaczur 1993].  Typically the electrosynthesis process is canied out in a two compartment electrolysis cell with a cation-exchange membrane as separator. Usually the anolyte is sodium hydroxide (sometimes sodium chloride), while the catholyte is a sodium bisulfite or sulfur dioxide solution. The cathode is usually a stainless steel or graphite electrode.  This is a complex reaction system. The possible major reactions are summarized in Table 2.5. In this Table, Reaction (1)  —  (3) are the possible main reactions; Reaction (4)  is the secondary reaction; Reaction (5) is the possible side reaction; Reaction (7)  —  (10)  are reactions of sulfur dioxide absorption into feed liquor to produce sodium bisulfite. Reaction (11)  —  (14) are the decomposition reactions of sodium dithionite.  The predominant species in sulfurous acid solution shift with pH. At 25°C, their distribution are roughly shown in the following [Oloman, et al. 1990]: 0<pH< 1.8,  0 3 1 S0 S 2 H  1.8<pH<6.9  3 HSO  6.9<pH<14  32 So 22  Chapter 2. Literature Review Table 2.5. Malor reactions in electrosynthesis of sodium dithionite [Olornan, et al. 19901 E° Volts SHE  Cathode electrochemical reactions 1. 2HSO  +  +  2. 2HS0 3  +  2H + 2e  —*  4 0 2 S  2e  -  4 0 2 S  0 2 3. 2SO 2 +2H  2e  +  4. HSO+ H + 2e 5. 2H 0 2  2e-  +  —*  —*  —*  2 H  +  - + 2H 4 0 2 HS 0 2  -0.07  +  0 2 2H  +0.10  +  40H  -1.13  3 0 2 S  +  +0.87  O 2 H  -0.83  20W  Anode electrochemical reactions 6. 02  +  0 2 2H  +  4e  +—  +0.40  40H  Bulk catholyte thermochemical reactions 7. SO 2 (g)  2 (aq) SO  —*  S0 2 8. SO 2 (ag)+H20 —*H 9. HSO 3  —*  10. HSO-  H + HSO  pKa=1.81 at 18 0 C  H + SO2-  pKa6.91 at 18 0 C  —*  O 2 4 +H 0 2 11.S  —*  O 2 S  12. S - + 2H 0 2  —*  2 SO  13. S 4 0 2  +  S2O2  4 +02 0 2 14. 2S  +  +  +  0 2 H  0 2 2H  +  2HSO  S +1120 —*  —*  2- +  -+2 2 so 0j S  +  2H  4HS0  In the main reactions, the real reactive species is sulfhr dioxide or sulfurous acid although bisulfite is the predominent species under weak acid condition [Koithoff and Miller 1941]. At low pH, dithionite formation proceeds more easily. However, its decomposition is also enhanced. So there is an optimum pH for this process.  In a cation-exchange membrane cell, several obstacles retard the efficient electrosynthesis of sodium dithionite, such as the decomposition of sodium dithionite,  23  Chapter 2. Literature Review secondary electrochemical reaction  —  cathodic reduction of dithionite to thiosulfate. The  latter predominates at high current density (above the critical current density) or high dithionite concentration because anyway Reaction (4) is thermodynamically more favorable than Reaction (2). The following process variables control current efficiency and yield of the process: cathode current density, ratio of cathode surface area to cathode chamber volume (sometimes current concentration is used instead of this ratio), process temperature, pH, catholyte velocity (in continuos process) and reactant concentration.  The biggest difficulty in efficient electrosynthesis of sodium dithionite is that sodium dithionite is unstable. Oxygen, low pH and transition metal ions enhance decomposition of sodium dithionite. In the pH range of 4.5 —7, higher concentrations of dithionite, bisulfite, thiosulfate and hydrogen ion also increase dithionite decomposition rate [Rinker et al. 1965; Spencer 19671. Generally, above pH 5, sodium dithionite is relatively stable under anaerobic condition. At ambient temperature, the half life of electrochemically generated sodium dithionite is about 10 hours at pH 5, the half life extends to 50 hours at pH 12, while in an acid environment (pH about 1), the half life plunges to a few minutes [Oloman et al. 1969; Oloman et al. 1990  ].  The temperature  effect on dithionite decomposition is not very big below about 50°C [Melzer 1990].  In order to tackle these problems, some engineering measures have been successfully employed to increase current efficiency, dithionite yield and product concentration. Inert atmosphere in the cell is used to reduce the decomposition rate. Higher ratio of cathode surface area to cathode chamber volume is critical because it raises the ratio of dithionite formation rate to dithionite decomposition rate. This will increase dithionite yield and current efficiency. But this ratio is restricted by the mechanical structure of the electrochemical cell. The best choice to date is to use a threedimensional cathode, such as packed bed cathode. With the increase in current density, 24  Chapter 2. Literature Review dithionite yield rises until a maximum yield is reached; at which, the rate of dithionite formation is equal to the rate of dithionite secondary reaction plus the rate of decomposition. Beyond it, the yield falls as hydrogen and sulfur are generated. Some processes operate in acid condition (pH around 1  —  3) and employ short residence time  (in the order of seconds) and sub-critical current density. Others operate at weak acidic environment (pH around 5  —  6) and employ long residence time and sub-critical current  density. Increases in cathode area will increase dithionite yield until a maximum is reached, at which dithionite formation rate is balanced by decomposition rate plus secondary reaction rate. Sulfur dioxide or bisulfite concentration should be maintained at a certain level so that the potential of Reaction (2) will be higher than that of Reaction (4), consequently secondary reaction will be minimized or even eliminated. In the batch process, product concentration can reach 16 wt% Na 4 at current efficiency of 80% O S 2 and yield of more than 80% [Anonymous 1937]. In the continuous process, product concentration can achieve 15 wt% at current efficiency of 90% [Bolick et al. 1988].  Up to date no process succeeds in operating under alkaline condition although dithionite decomposition would be drastically reduced. It is unlikely that this process can proceed in alkaline conditions because reaction (5) is thermodynamically more favorable than reaction (3). In Oloman’s study [Oloman, et al. 1990], no sodium dithionite is generated in alkaline conditions, pH about 11  —  12, with current density of 0.4  —  2.0  2 and gas phase sulfur dioxide concentration of 4—40 vol% in the feed mixture. kA/m  Process temperature usually is around ambient temperature or even lower, such as 5°C to suppress dithionite decomposition.  Sodium dithionite solution from the electrochemical process differs from the product of the thermochemical process, it may also contain sulfur, thiosulfate and 25  Chapter 2. Literature Review unreacted bisuffite or sulfur dioxide. Its brightening power is similar to that normally used. With 1 wt% electrolytic sodium dithionite on od pulp, 9— 10 % ISO brightness gain can be obtained on groundwood pulp under conventional brightening condition. Temperature, time and pH effects in the brightening process are also similar for sodium dithionite from both the thermochemical process and the electrochemical process [Oloman and Austin 1969].  26  Chapter 2. Literature Review 2.9 Factorial Desi2n and Analysis of Experiments  Factorial design of experiments is a way to quickly explore the experimental response surface. Its advantages include that it is an organized approach toward the collection and analysis of information so that it can obtain more information per experiment than unplanned approaches, it can exhibit information reliability in the light of experimental and analytical variation, it can also exhibit interactions among experimental variables [Murphy 1977].  In a two level factorial design, the low level and high level of each variable are coded as ‘-V and ‘±1’, e.g. in a two variable (coded as Xl and X2) factorial design, experimental design and analysis are listed as follows:  Table 2.6. Two variable factorial design Xl  X2  Response  +1  +1  Y1  +1  -1  Y2  -1  +1  Y3  -1  -1  Y4  The main effect of Xl  =  ( (Yl + Y2)  -  (Y3  +  Y4) )/2  ( (Yl + Y3) (Y2 + Y4) )/2 The interaction effect of Xl, X2 ( (Yl + Y4) (Y2 + Y3) )12  The main effect of X2  -  =  -  =  (15) (16) (17)  For duplicates where the responses are denoted U and W, the response error s is (18)  27  Chapter 2. Literature Review The pooled variance for k separate estimates of error sj each with r replicates is (19)  =Z(r- l)sj 2 s IY..(r- 1) 2 with v degrees of freedom:  (20)  vE(r-1)  The confidence intervals (CI) for main effects and interaction effects are calculated as: (21)  CI = Effect estimate ± t’sij 5:  Response error estimated with v degrees of freedom.  t’: Student’s t statistics with v degrees of freedom at stated confidence level. N: Number of factorial runs in the design. If the absolute value of effect is higher than  t’s/’,  then the effect of the  response variable is significant at the stated confidence level.  For a more complete reference of factorial design a book “Statistics for Experimenters” [Box et al. 19781 is recommended.  28  Chapter 2. Literature Review 2.10 Sequential Simplex Optimization  The sequential simplex method used in this study is a kind of evolutionary operation (EVOP). A simplex is a geometric figure with vertexes of one more than the number of dimensions of the factor space. It represents a tangential planar approximation to the response surface in the design region. By eliminating the vertex with the worst response, and then projecting the coordinates through the weight center of the remaining vertexes an equal distance beyond, a new simplex is formed. A new experiment will be carried out at this reflection vertex. The main advantage of simplex EVOP over classic factorial EVOP is that the former saves workload, such as, in the initial simplex design, the number of runs is less; also when moving into the next design region of factor space, simplex EVOP requires only one new experiment [Walter, et al. 19911.  29  Chapter 3. Experimental Apparatus and Methods Chapter 3  Experimental Apparatus and Methods  This study consisted of three parts, an investigation of potential reducing  chemicals, an investigation of dithionite brightening at 1% pulp consistency and an investigation of in-situ electrochemically generated dithionite brightening.  In the first part of the work, four reducing chemicals which had the possibility to be electrochemically synthesized were chosen. Initially preliminary experiments on these brightening chemicals were conducted. Brightening of mechanical pulp by these chemicals was performed under weak acidic and weak alkaline conditions, while other process variables were kept constant. However, no significant brightness gain was observed. Thus no further experiments were conducted to investigate the pH, brightening agent charge, time, pulp consistency and temperature effect on brightness gain.  In the second part of the work, brightening with sodium dithionite at 1% pulp consistency was conducted. Dithionite brightening with multiple runs on the same pulp and interstage washing, with multiple charges in a single run, with bisulfite and with some additives was conducted.  In the last part of the work, investigation of dithionite generation by electrochemical methods was conducted. The effect of four process variables, pH, current, sodium sulfite dose and temperature on dithionite concentration and current efficiency was studied. Based on this knowledge, in-situ electrochemically generated dithionite brightening was conducted at a pulp consistency of 0.8%. The effect of four process variables, pH, current, sodium suffite charge and temperature on brightness gain, time 30  Chapter 3. Experimental Apparatus and Methods average sodium dithionite concentration and final current efficiency was investigated. The effects of cathode area and 2-propanol were also investigated. Based on this knowledge, a sequential simplex optimization was carried out at 2% pulp consistency in hope of achieving a brightness gain of up to 20 % ISO.  All pulp used in this study was softwood TIvIP whose specification is given in Appendix B. The pulp was pretreated by washing with DTPA according to the following procedure: 1. the pulp was diluted from consistency of about 20% (when receiving) to consistency of 2—3%. 2. this pulp suspension was mixed with 0.25 wt% DTPA on od pulp, heated to 60—65 °C and stirred for 1 hour at this temperature range. 3. this pulp suspension was dewatered with a centrifugal dryer to the consistency of 20 —25%. 4. the resulting pulp was stored in plastic bags in small batches, and the bags were pressed to remove the air as much as possible and then sealed. 5. the pulp was frozen until fhture use.  3.1 Experimental Procedure and Apparatus for Sodium Dithionite Analysis  In this study, sodium dithionite was titrated by potassium hexacyanoferrate with methylene blue indicator under nitrogen. Using this titration method, there was no interference of decomposition products (sulfate, sulfide, sulfite, trithionate, thiosulfate), additives (sodium chloride, phosphates, soda, EDTA, urea), methanol, ethanol, acetone or  31  Chapter 3. Experimental Apparatus and Methods groundwood pulp. This method is also very accurate, the standard deviation is 0.3% for commercial sodium dithionite with 91% purity [De Groot 1967].  The principle of this method is that in alkaline solution: 3 6 2Fe(CN)  +  4 0 2 S  +  40H  —*  2 3 2S0  +  4 6 2Fe(CN)  +  0 2 2H  (22)  The titration apparatus was a 250 ml Erlenmeyer flask with a 3 opening stopper, for a nitrogen inlet tube, a burette and a nitrogen outlet tube. The nitrogen inlet tube was first connected to a gas flowmeter, and then to a nitrogen cylinder. The nitrogen outlet tube was connected to a gas trap to prevent air coming into the system. Thus a nitrogen blanket in the titration system was provided. Mixing of titration solution was provided by a stirrer. The apparatus used in this study is shown in Figure 3.1  The procedure consists of mixing in a 250 ml Erlenmeyer flask 45 ml distilled water, 5 ml 1M sodium hydroxide, 5 ml methanol, 0.25 ml 0.25 wt% methylene blue solution and 1.5 ml sodium zincate solution (if many analyses are to be made, a solution containing all these components proportionally can be freshly made everyday). A 5 ml or 10 ml liquor sample is taken from the reactor and transferred quickly to the flask with shaking. The blue color of the solution will fade if there is sodium dithionite present in the sample. The solution is titrated quickly with standardized 0.02 M or 0.1 M potassium hexacyanoferrate solution under nitrogen purge until the solution gets faintly blue.  The concentration of sodium dithionite (in g/l) can be calculated from the following formula: concentration of sodium dithionite (in g/l)  =  (concentration of potassium hexacyanoferrate (in M)) x (volume of potassium hexacyanoferrate (in ml)) x (174.1)/(2 x volume of sample (in ml)) 32  (23)  El.  .7  7  -\ I  Figure 3.1. Apparatus for dithionite titration inlet tube; 4. Burette; 5. Hot plate stirrer; 6. Nitrogen outlet tube; 7. Gas trap. 3. Nitrogen 1. Nitrogen cylinder; 2. Gas flowmeter;  6/  2  4  I  0  Chapter 3. Experimental Apparatus and Methods Reagent zincate solution was prepared according to the following procedure: A 70 2 was dissolved in 400 ml deionized water and 11 M NaOH solution was added g ZnC1 until the precipitate initially formed had dissolved again, then the zincate solution was made up to 11 with deionized water.  3.2 Conventional Brihtenin of Mechanical Pulp  A. Measuring Brightness and Yellowness ofUnbrightened Pulp  Pulp stock consistency was measured according to the procedures in Appendix A. The amount of pulp stock that was equivalent to 4 g od pulp, was weighed and disintegrated in standard disintegrator for 5 minutes in 2 1 deionized water. The pH of the suspension was measured and adjusted to the range of 6  —  7. A handsheet was made  according to the procedures set in CPPA Standard C.5, dried for 24 hours at 23°C and 50% RH (relative humidity), then folded twice to form a rectangular sector shape. The brightness and yellowness were measured with an Elrepho spectrophotometer at 2 spots upside and 2 spots downside of the handsheet. This test was repeated for another sample. The final brightness and yellowness of the unbrightened pulp was the average of these two results. This was done in the beginning of each series of experiments.  B. Brightening of Mechanical Pulp  The apparatus used in this study is shown in Figure 3.2. The brightening reactor was a 1500 ml K]MAX glass beaker whose inside diameter and depth were respectively 118 and 160 mm.  34  Figure 3.2. Apparatus for conventional brightening Mixer; 6. Brightening reactor; 1. Nitrogen cylinder; 2. Gas flowmeter; 3. Nitrogen inlet tube; 4. pH meter with “3 in 1” pH electrode; 5. 7. Nitrogen outlet tube; 8. Water bath; 9. Gas trap.  (P  (p  >  -i  Chapter 3. Experimental Apparatus and Methods An acid buffer was prepared with 0.2 M sodium acetate and 0.45 M acetic acid, and an alkaline buffer with 0.2 M sodium hydroxide and 0.05 M sodium tetraborate.  The designated amount of pulp stock was weighed, disintegrated in standard disintegrator for 5 minutes in 2 1 deionized water, then dewatered in a standard handsheet maker. The resulting handsheet was torn into 10 mm x 10 mm pieces.  Water bath was turned on and the temperature was set at the required value.  The pieces of handsheet were added to the brightening reactor plus an appropriate amount of buffer solution so that the final weight of the brightening system would be the designated one, such as 400 g (the volume of brightening system would be close to 400 ml), STPP or EDTANa 2 was added, then the suspension was mixed until it became even. The pH was adjust to the designated value ± 0.1 pH units by 1 M sulfuric acid solution and/or 1 M sodium hydroxide solution, then the suspension was heated to reaction temperature and the reactor was moved into the water bath. The reactor mixer and nitrogen were turned on so that the brightening reaction would be under a nitrogen blanket, and the nitrogen flow rate was set at about 5 1/mm STP. After a few minutes, the temperature of the pulp suspension became stable, brightening agents were added to the reactor, and the brightening began.  When the designated reaction time was reached, the brightening ended. The nitrogen and reaction mixer were turned off, the reactor was taken out of the water bath, then the pulp suspension was transferred from the reactor into about 1 liter of deionized water to quench the brightening reaction. The pulp suspension was mixed thoroughly, transferred to the standard handsheet maker, and dewatered. Two pieces of blotting paper were placed on top of the pulp, a standard couch plate on top of the blotting paper, and a 36  Chapter 3. Experimental Apparatus and Methods standard couch roll in the middle of the plate, then the couch roll was rolled a few times and lifted. The handsheet was taken off, mixed with 11 deionized water in a beaker until the suspension became even. This dewatering and mixing procedure were repeated twice. The pH of the suspension was measured and adjusted to the range of 6—7. A handsheet was made according to the procedures set in CPPA standard C.5, dried and the brightness and yellowness were measured as described in Part A above.  The procedure of brightening by potential brightening agents was the same as that by sodium dithionite, except the reaction volume was 500 ml. The apparatus of brightening by potential brightening agents was the same as that shown in Figure 3.2.  3.3 Electrochemical Generation of Sodium Dithionite  The apparatus used in this study was the same as that for “brightening mechanical pulp by in-situ generation of sodium dithionite”, as shown in Figure 3.3. The electrochemical reactor used in this study was a 316 stainless steel electrochemical reactor whose inside wall was coated with KYNAR® (a corrosion resistant coating). A test was carried out, and no corrosion was observed in this reactor during electrochemical synthesis of dithionite. This reactor consisted of two half cylinder compartments (anode and cathode compartment) with inside diameter of 160 mm and depth of 133 mm, separated by a Naflon 324 cation exchange membrane which was supported by two perforated Plexiglas plates. The compartments, neoprene gaskets, perforated plates and membrane were bolted together at the outside flange of the reactor. In this study, the anode was a rectangular platinized titanium plate, the cathode was a stainless steel 316 screen. The screen was “mesh 60, 0.1905 mm (0.0075”) diameter wire”, its ratio of real surface area to superficial surface area was 1.228, as shown in Appendix C. The cathode had two configurations, rectangular and half cylinder, while anode had only one configuration, rectangular. For the 37  Chapter 3. Experimental Apparatus and Methods rectangular cathode whose width and length were 100 and 118 mm, the corresponding anode had the same width and length, and for the half cylinder cathode whose diameter and length were 150 and 118 mm, the corresponding anode had width of 13 mm and length of 118 mm. The anode and cathode were immersed to a depth of 45 mm in electrolyte. These immersed cathode areas are shown in Table 3.1. Table 3.1. Cathode areas immersed in pulp suspension Rectangular Cathode Superficial Area  Real Area  100mm  *  Half Circular Cathode 1/2*3.14*150mm  45mm  2 =4500mm  2 mm=10600mm  5500 mm 2  13000 mm 2  *  45  The anolyte was a 500 ml sulfuric acid solution whose concentration was chosen in such a way to minimize the fluctuation of catholyte pH during the reaction.  The designated amount of chemicals, such as sodium sulfite, EDTANa 2 (and sometimes sodium sulfate), were weighed and dissolved in 400 ml deionized water, the pH of the resulting solution was adjusted by sulfuric acid and/or sodium hydroxide. The catholyte was prepared by making up the volume of the above solution to 500 ml with deionized water. Both catholyte and anolyte were heated, and anolyte was transferred to the anode chamber and catholyte to the cathode chamber. Two reactor lids were put on, and clamped by two 3” C clamps each. The reactor was already in the water bath, which had been set at the appropriate temperature. The nitrogen and cathode chamber mixer were turned on, a few minutes was allowed to stabilize the catholyte temperature, catholyte pH was adjusted to the designated value with sulfuric acid and/or sodium hydroxide, the DC power was turned on with current at the designated value to start the reaction.  38  Chapter 3. Experimental Apparatus and Methods  The change of catholyte pH during the reaction was suppressed by addition of 1 M sulfuric acid and/or 1 M sodium hydroxide to keep the pH close to the designated value ± 0.3 pH units. Both catholyte and anolyte were under nitrogen purge, the nitrogen flow rates were 1.1 and 4.0 1/mm  STP, respectively. Samples of eatholyte were also taken  during the reaction to analyze for the concentration of sodium dithionite. When the reaction ended, power supply, nitrogen, water bath power and cathode chamber mixer were turned off, the clamps were unscrewed, the reactor lids were taken off, the anolyte  and the catholyte were dumped.  The formula for calculating current efficiency of dithionite is listed below:  CE=—’ I  (24)  R=Kc  (25)  I  CE  current efficiency  R  average reaction rate, mole/sec  I  current, Amp  V  reaction volume, 1  C  final concentration of dithionite, mole/i  t  reaction time, Sec.  n  electron stoichiometry coefficient (n =2)  F  Faraday constant  39  Chapter 3. Experimental Apparatus and Methods 3.4 Brihtenin Mechanical Pulp by In-situ Generation of Sodium Dithionite  Brightening mechanical pulp by in-situ electrochemical generation of sodium dithionite was extensively explored in this study. There were two reactors used in this study, one was made of Plexiglas, the other was made of stainless steel. The apparatus is shown in Figure 3.3.  The Plexiglas reactor was used at ambient temperature. It consisted of two half cylinder compartments (anode and cathode compartment) with inside diameter of 153 mm and depth of 155 mm, separated by Nation 214 cation exchange membrane which was supported by two perforated Plexiglas plates. The compartments, neoprene gaskets, perforated plates and membrane were bolted together at the outside flange of the reactor. In the study of electrochemical dithionite brightening at ambient temperature, the anode was a rectangular stainless steel 304 plate, the cathode was a rectangular stainless steel 316 screen. The screen was “mesh 8, 0.63 5 mm (0.025”) diameter wire”, its ratio of real surface area to superficial surface area was 0.656, as shown in Appendix C. The width and length of both anode and cathode were 100 and 125 mm, electrodes were immersed in electrolyte to a depth of 15 mm, thus the immersed cathode superficial area was 1500 , the immersed cathode real area was 1000 mm 2 mm . 2  The reactor used at elevated temperature was the stainless steel electrochemical reactor mentioned in Section 3.3. The cathodes used in this study were the rectangular and half circular cathodes made of screen “mesh 60, 0.1905 mm (0.0075”) diameter wire”, as described in Table 3.1.  The consistency of the pulp stock as well as the brightness and yellowness of the unbrightened pulp were measured as described in Part A of Section 3.2. 40  ::‘.  /1  /  IL.LrI.JJJLJJIj  10  iI  4  :>:  Ii I! II:  2  Figure 3.3. Apparatus for brightening mechanical pulp by in-situ generation of sodium dithionite 1. Nitrogen cylinder; 2. Gas flowmeter; 3. Nitrogen inlet tube; 4. Mixer; 5. pH meter with “3 in 1” pH electrode; 6. DC power supply;. 7. Cathode; 8. Anode; 9. Electrochemical reactor; 10. Nitrogen outlet tube; 11. Water bath; 12. Gas trap.  6  CD  CD  CD  Chapter 3. Experimental Apparatus and Methods The designated amount of pulp stock was weighed, disintegrated in a standard disintegrator for 5 minutes in 2 1 deionized water, the obtained suspension was dewatered in standard handsheet maker, and the resulting handsheet was torn into 10 mm x 10 mm pieces.  The anolyte was a 500 ml sulfuric acid solution whose concentration was chosen in such a way to minimize the pH fluctuation in the pulp suspension.  The designated amount of chemicals were weighed, such as sodium sulfite, 2 (and sometimes sodium sulfate, iso-propyl alcohol and chromium (III) nitrate EDTANa nonahydrate), dissolved in 400 ml deionized water and the pH of the solution was adjusted by sulfuric acid and/or sodium hydroxide. The catholyte was prepared by heating this solution to the required temperature (if the brightening was conducted at elevated temperature), and adding the pieces of handsheet along with the deionized water to make up the suspension volume to 500 ml.  If the brightening was conducted at elevated temperature, the anolyte would be heated during the period of catholyte preparation.  The catholyte was mixed until it became even and heated again if necessary. The anolyte was transferred to the anode chamber and catholyte to the cathode chamber. Two reactor lids were put on and clamped by two 3” C clamps each. The reactor was already in the water bath, which had been set at the designated temperature. The nitrogen and cathode chamber mixer were turned on, a few minutes was allowed to stabilize the catholyte temperature, catholyte pH was adjusted with sulfuric acid and/or sodium hydroxide, the DC power was turned on with current at the required value to start the reaction. 42  Chapter 3. Experimental Apparatus and Methods  During the reaction, catholyte pH was controlled by addition of 1 M sulfhric acid and/or 1 M sodium hydroxide and held close to the designated value ± 0.3 pH units at pulp consistency of 0.8% and 1%. In runs at 2.0% pulp consistency the pH was held within ± 0.4 pH units of the designated value. Both catholyte and anolyte were under nitrogen purge, the nitrogen flow rates were 1.1 and 4.0 1/mm STP, respectively. Samples of catholyte were also taken during the reaction to analyze for the concentration of sodium dithionite.  When the reaction ended, the power supply, nitrogen, water bath power and cathode chamber mixer were turned off, the clamps were unscrewed, the reactor lids were taken off, the anolyte was siphoned away and the catholyte was poured into 11 deionized water. If the pulp consistency was 0.8% or 1.0%, the resulting pulp suspension was mixed immediately and all of it was taken; if the pulp consistency was 2.0%, the suspension volume was made up to 2 1 with deionized water, the obtained suspension was mixed until it became even, and then 800 ml of it was taken. The resulting pulp suspension was transferred into a standard handsheet maker and dewatered. Two pieces of blotting paper were placed on top of the pulp, a standard couch plate on top of the blotting paper, a standard couch roll in the middle of the plate, and the couch roll was rolled a few times and then lifted. The handsheet was taken off, redissolved in 11 deionized water, and the pulp suspension was mixed. This dewatering and mixing step were repeated twice, finally the pH of pulp suspension was adjust to 5.0  —  5.5, a handsheet was made according to  the procedures set in CPPA standard C.5, dried and the brightness and yellowness were measured in a way described in Part A of Section 3.2.  43  Chapter 3. Experimental Apparatus and Methods 3.5 Software  The factorial experiments in this study were analyzed by JASS (version 2.0). JASS is a computer program used to design and analyze two level factorial experiments. It was -  developed by Joiner Associates. The sequential simplex optimization in this study was conducted by using OPTIMA (version 3.0). OPTIMA is a computer program for experimental optimization using the composite modified simplex method, an advanced algorithm for simplex optimization. It was developed by Wentzell, P.D. and Wade, A.P. of Department of Chemistry, University of British Columbia.  44  Chapter 4. Experimental Results and Discussion Chapter 4  Experimental Results and Discussion  4.1 Preliminary Investi!ation on Mixin2 Ouallty versus Pulp Consistency in Laboratory Reactor  A pulp suspension is a continuous fiber network with structure and strength resulting from interaction between neighboring fibers. Cohesive forces in pulp suspension arise from bending and hooking of fibers when the consistency of the suspension is just above 0.5 wt% [Kerekes et al. 1985]. In industry, pulp consistency of up to 6% is called low consistency. In most bench scale brightening and bleaching experiments, pulp consistency is 3  —  4% and there is no mixing during reaction.  In order to determine the suitable pulp consistency in this study a simple experiment was performed. The experimental conditions were as follows. The reaction vessel was a 2000 ml KItvfAX glass beaker with inside diameter of 124 mm. A Plexiglas plate of 105 mm was inserted as a separator to make two chambers in the beaker, and the smaller chamber was about the same size of the proposed electrochemical reactor. The plate did not fit the beaker perfectly, so there was some opening between these two chambers. The mixer was a 40 W (1/75 HP) Tline Laboratory Stirrer with a 48 mm diameter propylene propeller. The distance between the propeller and the vessel was 13 mm, and the suspension volume was 500 ml. The motor power used in this study was 50% of the maximum power allowed. The method to assess mixing quality was to observe how a dye spreads in the suspension, the mixing quality was observed with increases in pulp consistency and the results are summarized in Table 4.1. A consistency of 2% was finally  45  Chapter 4. Experimental Results and Discussion chosen as the upper limit for this study. In order to have good mixing a consistency of around 1% was used in most of this study.  Table 4.1. Mixing quality versus pulp consistency in simulated laboratory reactor Pulp Consistency  Mixing Quality  1%  good  2%  OK  4%  poor  46  Chapter 4. Experimental Results and Discussion  4.2 Preliminary Investigation of Possible Bri2htenin Agents  A preliminary investigation was conducted to examine new reductive brightening agents. Four reductants were tried and their main industrial applications are listed in Table 4.2.  Table 4.2. Reductive brightenin, agents in trial Reductants  Applications  N NH . S 2 ) H I{ 0 Hydrazine sulfate ( 4  Silver-plating of glass and plastics  Hydroguinone ( 2 -1,4-(OH) 4 H 6 C )  Photographic developer  Hydroxylamine sulfate 0 (NH . S 2 ) H H) 0 (4  Reduction of cupric salt in dyeing acrylonitrile fibers  PO) 2 Sodium hypophosphite ( NaH  Electroless nickel plating  The optimum conditions for reductive mechanical pulp brightening are probably high temperature (about 60°C), appropriate reaction period (1 hour), with chelating agent 2 blanket to avoid decomposition of the brightening agent to complex metal ions and N plus a high charge* of brightening agent (about 4 wt% on od pulp) and most likely under weak acidic condition, but possibly under weak alkaline condition. Since this set of  experiments was highly speculative, no electrochemical brightening was conducted unless promising results were obtained in these preliminary tests.  *  In this thesis, the charge of brightening agent and chelating agent are listed as weight  percentage on od pulp  47  Chapter 4. Experimental Results and Discussion A. Hydrazine Sulfate  Table 4.3 shows that under weak acidic condition, hydrazine sulfate darkens mechanical pulp severely in 45 minutes. It is inferred that alkaline condition will darken mechanical pulp even more severely. So it is unlikely that hydrazine sulfate will be a strong brightening agent.  Table 4.3. Hydrazine sulfate brightening Run No.  Operating  Results  Conditions BG (% ISO) charge*l 1% consistency /4% A1.1 /0.5% EDTANa 2 *2 /pH 5.5 -15.1 1T60°C/ 45 mm / N Original pulp (TIvIP 1, specification in Appendix B): B (Brightness) = 49.9 % ISO, Y(Yellowness) = 30.5 %. *1 :4wt%onodpulp. *2: 0.5 wt% on od pulp.  YL  (%)  -30.9  B. Hydroguinone  Table 4.4 shows that hydroquinone brightens mechanical pulp by up to 4% ISO under weak acidic condition, and darkens mechanical pulp in just 20 minutes under weak alkaline condition. So it is unlikely that hydroquinone will be a strong brightening agent.  48  Chapter 4. Experimental Results and Discussion Table 4.4. Hydroquinone brightening Run No.  A2.l  Operating  Results  Conditions  BG (% ISO)  YL  2% consistency /4% charge 2 /pH 5.0 /T=60°C/ /0.5% STPP*  3.6  0.9  -0.9  -1.9  1 hr / N 9 1% consistency /4% charge /0.5% STPP /pH 10.0 /T=60°C/ 20 mm / N 9  A2.2  Original pulp (TMP 1): B  =  (%)  49.9% ISO, Y = 30.5 %.  *2: 0.5wt°honodpulp.  C. Hydroxylamine sulfate  Table 4.5 shows that hydroxylamine sulfate brightens mechanical pulp by up to 3% ISO under both weak acidic and weak alkaline conditions. High charges (16%) only brighten pulp by 1.0 % ISO. So it is unlikely that hydroxylamine sulfate will be a strong brightening agent.  Table 4.5. Hydroxylamine sulfate brightening Run No.  A3.1 A3.2 A3 .3  Operating  Results  Conditions  BG (% ISO)  YL  3.4  1.9  0.0  -1.9  1.0  -1.1  1% consistency /4% charge /0.5% STPP /pH 4.0 Ir=60°C/ 1 hr / N 9 1% consistency /4% charge /0.5% STPP /pH 9.4 /T=60°C/ 1  hr/N 9 1% consistency /16% charge /0.5% STPP /pH 5.5 /T=60°C/ 9 20mm/N  Original pulp (TMP 1): B  =  -  49.9% ISO, Y  49  =  30.5 %.  (% ISO)  Chapter 4. Experimental Results and Discussion D. Sodium Hvpophosphite  Table 4.6 shows that sodium hypophosphite brightens mechanical pulp by up to 4% ISO under weak acidic condition. The effects of temperature, pH and charge on sodium hypophosphite brightening are not significant under weak acidic condition. Sodium hypophosphite probably will not achieve higher brightness gain under weak alkaline condition than under weak acidic condition because its structure and chemical properties are similar to those of sodium dithionite. So sodium hypophosphite probably will not be a strong brightening agent.  Table 4.6. Sodium hypophos bite brightening Run No.  A4. 1  A4.2 A4.3 A4.4  Operating Conditions  Results  1% consistency /4% charge /0.5% EDTANa 2 /pH 4.0 /T70°C/ 1 hr / N 7 1% consistency /4% charge /0.5% EDTANa 2 /pH 4.0 /T=50°C/ 1 hr / N 7 1% consistency /4% charge /0.5% EDTANa 2 /pH 5.5 /T=50°C/ 1 hr / N 7 1% consistency /16% charge /0.5% EDTANa 2 /pH 5.5 rr=so°ci 1 hr / N 7  Original pulp (TMP 1): B  =  B.G (% ISO)  YL (%)  3.2  0.2  3.8  1.1  3.4  0.9  3.1  0.3  49.9% ISO, Y = 30.5 %.  E. Summary  These four reducing agents only brightened mechanical pulp by up to 4 % ISO, they seemed less effective in brightening mechanical pulp than sodium dithionite, thus no experiments on electrochemical brightening by these agents were conducted.  50  Chapter 4. Experimental Results and Discussion 4.3 Sodium Dithionite Brightening at 1% Pulp Consistency  Sodium dithionite is widely used in mechanical pulp brightening. The usual industrial operating conditions are as follows: 3 and 0.2-0.5% S’PP charge on od pulp, pH 5.0  —  —  4% pulp consistency, 1% dithionite 5.5, temperature 60°C, brightening  period 1 hour and exclusion of air. In the present study, it was impossible to achieve good mixing in the laboratory apparatus at even low pulp consistency, 3 —4%, as the previous study of Section 4.1 revealed. So investigation on dithionite brightening at very low consistency, 1% was conducted to obtain the brightness response under this specific condition. In this study, the total reaction volume was 400 ml and the pulp suspension was not buffered.  Brightness responses versus dithionite charges are listed in Table D.1 of Appendix D and plotted in Figure 4.1. The maximum brightness gain is 10.7 % ISO, and this is at the same level as the brightness gain obtained by sodium dithionite at 3 —4% consistency in standard laboratory practice. Beyond a certain charge, in this case 1%, further increases  in dithionite charge can not improve brightness. The reason could be that dithionite will attack only certain chromophores. Once these chromophores are consumed excess dithionite has no effect.  51  Chapter 4. Experimental Results and Discussion  65•  I  55.  1• •  0  1  2  3  •  4  •  5  •  6  •  7  8  Charge (% on od pulp)  Figure 4.1. Brightness responses of dithionite brightening versus charge at 1% pulp consistency (0.5% STPP/ pH 5.0/ T  60 °C/ 60 mm.! exclusion of Air)  52  Chapter 4. Experimental Results and Discussion A more comprehensive set of experiments was conducted to investigate the brightness responses under multiple charge (e.g. 2% on od pulp) dithionite brightening. In this experiment, the total reaction volume was 400 ml and the pulp suspension was prepared from an acidic pH buffer solution described in Part B of Section 3.2.  Results from multiple charge dithionite brightening at ambient temperature are listed in Table D.2 of Appendix D and plotted in Figure 4.2. At ambient temperature, multiple addition of dithionite (4 additions, each of 2% charge in 15 mm. interval) only slightly improves the brightness gain than single addition of dithionite. However, the brightness gain is still much lower than that obtained from single addition of dithionite at 60°C, which is shown in Figure 4.1.  1%H  4”2%H Charge  Figure 4.2. Multiple charge dithionite brightening at ambient temperature (H—sodium dithionite, 4*_four times addition) (1% consistency /2% STPPI pH 5.5/T 20 °C/ 60 minJN ) 2  53  Chapter 4. Experimental Results and Discussion Multiple charge dithionite brightening at elevated temperature was also conducted. The results are listed in Table D.3 of Appendix D and plotted in Figure 4.3. The standard deviation of this set of experiment, which is 0.4 % ISO, is estimated from two replicated runs, Run Nos. B3 and B4, with an average brightness gain of 9.1 % ISO. Figure 4.3. shows that multiple stage dithionite brightening with interstage washing can achieve slightly higher brightness gain (about 1% ISO) than multiple addition in a single run with the same amount of total charge, and multiple addition can achieve slightly higher brightness gain (about 1 % ISO) than single addition with same amount of total charge, if acknowledging that in single charge dithionite brightening, brightness gain levels off beyond 1.0 % charge of dithionite on od pulp, as Figure 4.1 reveals. However, the time and the amount of each charge do not seem to affect the results of multiple charge brightening.  The experimental results reveal that multiple stage brightening with interstage  washing is very effective to achieve high brightness gain. This may be because dithionite brightening potential decreases in the presence of large amount of sulfite, or bisuffite as well as other reaction products of dithionite. This may help explain why the time and the amount of each charge have no effect on brightness gain in multiple charge brightening. Dithionite in later addition(s) probably loses its brightening potential due to the presence of bisuffite and other dithionite reaction products. With interstage washing, little dithionite reaction products are left and the newly added dithionite has its high brightening potential to further increase the brightness.  54  Chapter 4. Experimental Results and Discussion  0  Charge  Figure 4.3. Multiple charge dithionite brightening at elevated temperature (H—sodium dithionite, W—washing, 2*, 4*_ 2, 4 times addition) (1% consistency! 2% STPP/ pH 5.5! T 60 °C! 60 mm.! N ) 2  Multiple charge dithionite brightening with sodium sulfite (which exists as bisulfite under experimental conditions) at elevated temperature was thus tried to verif’j the above explanation, and the results are listed in Table D.4 of Appendix D and plotted in Figure 4.4. When dithionite brightening is conducted with high sodium sulfite charge, the brightness gain achieved is significantly lower than that with sodium dithionite alone. This supports the claim that sodium bisulfite decreases the brightening potential of dithionite. The fact that in the presence of 131 % sulfite on od pulp, dithionite still has some brightening potential and a very big increase in sulfite charge from 13.1 % to 131 % on od pulp only lowers about 1 % ISO brightness gain contributed by dithionite indicate that some other dithionite reaction products, i.e. thiosulfate and suffide may play a role in decreasing sodium dithionite brightening potential.  55  Chapter 4. Experimental Results and Discussion  10  .  9  0%H 42%H  8  7  q5 4 3 2  0 0%S  i3.i%S  131%S  SuIi Charge  Figure 4.4. Multiple charge dithionite brightening with sodium sulfite at elevated temperature (H—sodium dithionite, S—sodium suffite, 4*_four times) (1% consistency! 2% STPP/ pH 5.5/ T =60 °C! 60 mm.! N ) 2  In order to suppress this negative effect of sulfite on dithionite brightening, 2propanol, methanol, silver nitrate and formaldehyde were tried, and the results are listed in Table D.5 of Appendix D and plotted in Figure 4.5. Among the chemicals tried, only the first two can alleviate this negative effect.  56  — o C)  C/)  LS  —.  B.G.  0  0-  0  I  CD  r3 I  C)  .  I  01 I  0) I  J  co  CD  0  I  None -I  *  0*  1O%F  0  II. cDS:.  I0  LI)c  o E  0 C -‘  nI  E O.3%N  0 a  w  CD CD  r’J  2O0/4  .  0  JD  ‘:4 o  UO!SSflS!G  pu snsa’>j 1uuiudx3  Chapter 4. Experimental Results and Discussion 4.4 Electrochemical Generation of Sodium Dithionite at Elevated temperature  In order to tell the feasibility of in-situ electrochemical brightening with sodium dithionite at elevated temperature, electrochemical generation of dithionite at elevated temperature (without pulp) was first scrutinized. Although it is well documented that sodium dithionite can be synthesized electrochemicafly at ambient or lower temperature, no work has been done on electrochemical synthesis of sodium dithionite at elevated temperature, such as 40—60 °C. Thus an experiment was conducted to study the effect of process variables, such as pH., temperature, sodium suffite dose and current on time average dithionite concentration and final current efficiency. The effect of the two cathode configurations described in Section 3.3 was also investigated.  A. Electrochemical Synthesis of Dithionite at Elevated Temperature with Rectangular Cathode  Electrochemical generation of sodium dithionite at elevated temperature with the rectangular cathode was investigated. A 3 level, 4 factor factorial experiment was carried out with the following factors. Current  I  Sodium Suffite Dose*  C  pH  pH  Temperature  T  *Suffite is added as sodium sulfite, but exists in solution predominantly as sodium bisulfite.  The process conditions, symbols, experimental variables and results are listed in Table D.6, D.7 and D.8 of Appendix D. High time average concentrations of sodium dithionite, 58  Chapter 4. Experimental Results and Discussion 5 to 7 g/l, which is about 60—90 wt% on od pulp, are obtained at high current ( 2.25 A) together with high sulfite dose ( 25.3 g). Final current efficiencies for dithionite under these conditions are also fairly good, 35  —  60 %. However, no dithionite is obtained at  low sulfite dose (4.6 g), no matter what the level of the other variables.  Sodium sulfate is used to increase the electric conductivity of the solution. The effect of sodium sulfate on this dithionite synthesis process was also examined. Comparison at two process conditions was made. The experimental variables and results are shown in Table D.9 of Appendix D. This table shows that sodium sulfate has no significant effect on electrochemical synthesis of sodium dithionite (less than 10% difference on time average dithionite concentration) at high sodium sulfite dose.  The results of the factorial analysis are listed in Table D. 10 of Appendix D. The standard deviation of time average dithionite concentration is 0.08 g/l. The main factors C, I and IxC have the most significant effects on time average dithionite concentration, listed in declining sequence, and increases in C and I will increase time average dithionite concentration. Although the calculated standard deviation of final current efficiency is 0.00%, this is not considered to be a realistic estimate of standard deviation, so the interactive effect of four factors is taken as a estimate of the standard deviation. The main factor C has the most significant effect on final current efficiency, increases in C will increase final current efficiency, and the effect of C on final current efficiency and time average dithionite concentration has the same sign.  59  Chapter 4. Experimental Results and Discussion B. Electrochemical Synthesis of Dithionite at Elevated Temperature with Half cylinder Cathode  The half cylinder cathode had 2.4 times the area of the rectangular cathode. With a bigger cathode area, it was possible to generate dithionite at low sodium sulfite dose. Investigation of electrochemical synthesis of sodium dithionite at elevated temperature with the half cylinder cathode was conducted. A 3 level, 4 factor (pH, T, C and I) factorial experiment was carried out. The operating conditions, symbols, experimental variables and results, and two pairs of repeated runs are listed in Tables D. 11, D. 12, D. 13 and D. 14 of Appendix D. High time average sodium dithionite concentrations, 6 to 10 g/l (about 80 to 130 wt% on od pulp), are obtained at high current ( 2.25 A) together with high sodium sulfite dose ( 25.3 g), and final current efficiencies under these conditions are also fairly good, 50—80 %. However, no dithionite is obtained at low suffite dose (4.6 g) together with high current (4.0 A). Also the time average dithionite concentrations are very low (< 0.1 g/l) at a combination of low current (0.5 A), low sulfite dose (4.6 g) and low pH (4.0).  The standard deviations in this experiment are estimated from two pairs of repeated runs listed in Table D.14. The formula used is the square root of the average  square of the experimental errors. The standard deviations of time average dithionite concentration and final current efficiency are 0.05g11 and 0.6%, respectively.  The results of the factorial analysis are listed in Table D. 15. The main factors C, Ix C and I have the most significant effects on time average dithionite concentration, listed in declining sequence. Increases in C and increases in I at high suffite dose will increase time averaáe dithionite concentration. Decreases in T will also increase time average dithionite concentration slightly. The main factors C and I also have the most significant effects on final current efficiency, listed in declining sequence. Increases in C or decreases in I will 60  Chapter 4. Experimental Results and Discussion increase final current efficiency. The effect of I on final CE and time average dithionite concentration has opposite sign, while the effect of C on final CE and time average dithionite concentration has the same sign.  Both the time average dithionite concentration and final current efficiency are significantly higher in dithionite synthesis with the half cylinder cathode than those in dithionite synthesis with the rectangular cathode, especially at high sulfite dose. At both high current and high sulfite dose, time average dithionite concentration and final current efficiency with the half cylinder cathode are 20  —  110% and 30  —  130 %, respectively  higher than those with the rectangular cathode. This shows that cathode area is also a very significant factor on electrochemical synthesis of dithionite, and increases in cathode area will improve this synthesis process.  61  Chapter 4. Experimental Results and Discussion 4.5 In-situ Electrochemically Generated Dithionite Briahtenin at Ambient Temperature  Preliminary investigations on in-situ electrochemically generated dithionite brightening of softwood TMP at ambient temperature were conducted. The Plexiglas reactor with rectangular cathode described in Section 3.4 was used for these tests. Operating variables and experimental results are listed in Table D.16 of Appendix D.  In this study, the standard deviation of the brightness gain (0.2 % ISO) is estimated from two pairs of repeated runs: Runs Cia, Cib and Runs C7a, C7b. The average brightness gain of Runs Cia, Clb and Runs C7a, CTh are 4.1 and 3.1 % ISO. Figures 4.6 and 4.7 show the typical time effect on sodium dithionite concentration and pH in in-situ electrochemically generated dithionite brightening. The only difference in operating variables of Run C2 and Run Cia, as well as Run C4 and Run C3 is brightening time, Run Cia and Run C3 have longer brightening time —60 minutes, while Run C2 and Run C4 have only 10 minutes, thus the final dithionite concentration in Run C2 and Run C4 can be estimated from Run Cia and Run C3 at t  =  10 minutes, and they are 0.44 and  0.85 g/l respectively. The results of some selected runs are plotted in Figure 4.8. This figure shows that at ambient temperature, generally brightness gain is 3  —  5 % ISO,  which is at the same level as the run without current. Brightening time and current, or electrochemically generated dithionite do not seem to further increase the brightness upon that achieved by (bi)sulfite, even though in some runs, dithionite always exists during the brightening, and the final dithionite concentration is equivalent to 24 wt% on od pulp. Compared to the single and multiple charge dithionite brightening (without bisulfite) at ambient temperature, the brightness gain is still at the same level. In order to achieve higher brightness gain, further electrochemical dithionite brightening will be carried out at elevated temperature. 62  Chapter 4. Experimental Results and Discussion  I  ‘  I  •  I  I  •  I  •  •  1.0  —A—-COflC.  6.1•  —0— pH  0.8  6.0•  5.9  -  /  /  /  /  /  60:: •  •  Time(min.)  Figure 4.6. Sodium dithionite concentration and pH profile in electrochemical dithionite brightening at ambient temperature. SO dose = 26.3 g/ Initial pH = 5.5/ 1.0 A /60 2 (Na Run Cia: 3 miii..! 20°C)  From this figure, sodium dithionite concentration at 10 minutes is 0.44 g/l.  63  Chapter 4. Experimental Results and Discussion  •  •  3.0  •  •  •  5.82.5 5.6 •  /  -  2.0  7/ //  5.0•  II  I  I  /  O0  -1.o  O  • •  ‘  •  Time (mm.)  Figure 4.7. Sodium dithionite concentration and pH profile in electrochemical dithionite brightening at ambient temperature. SO dose = 26.3 g/ Initial pH = 4.5/ 1.0 A /60 mm.! 2 (Na Run C3: 3 20°C)  From this figure, sodium dithionite concentration at 10 minutes is 0.85 g/l.  64  Chapter 4. Experimental Results and Discussion  58—•—I=1.OA —K—-I=O.1A —A—I=O.OA O.P.B.  Brightening Time (mm.)  Figure 4.8. Brightness responses of electrochemically generated dithionite brightening versus brightening time (sodium sulfite dose=26.3 g/ initial pH=5.5/T=20 OC). O.P.B. = Original Pulp Brightness (Pulp has already been brightened from 52.4 % ISO (original pulp brightness) to 55.3 % ISO in the preparation procedure.)  65  Chapter 4. Experimental Results and Discussion 4.6 In-Situ Electrochemically Generated Dithionite Brihtenina at Elevated Temperature  Brightening mechanical pulp by in-situ electrochemically generated sodium dithionite was extensively explored in the present study. In the cathode chamber, sodium bisulphite is reduced to sodium dithionite, which brightens pulp. Factorial designs were used to investigate the effects of several operating variables on brightness gain and current efficiency for dithionite generation. The two cathode configurations described in Section 3.3 were also investigated.  A. Low Sodium Sulfite Dose Electrochemical Brightening at Elevated Temperature with Half Cylinder Cathode  Previous experiments in Section 4.4 showed that with the half cylinder cathode at elevated temperature, dithionite could be generated electrocheniically at low suffite dose together with low current. Thus investigation of electrochemically generated dithionite brightening at elevated temperature with the half cylinder cathode at low suffite dose together with low current was conducted. A 2 level, 2 factor (pIT, T) replicated factorial experiment was carried out.  The operating conditions, symbols, experimental variables and results are listed in Tables D.17, D.18 and D.19 of Appendix D. Only at pH=5.5 together with T=40°C, dithionite is obtained, the run average value of the time average dithionite concentration is equivalent to about 2 wt% on od pulp, and the run average value of the brightness gain is 4.1 % ISO with final current efficiency for dithionite of 7.2%. This brightness gain is lower than that of the conventional dithionite brightening at 1% pulp consistency with 1 wt% dithionite charge at 60°C. The highest run average value of the brightness gain, 6.8 66  Chapter 4. Experimental Results and Discussion % ISO, is obtained at pH=5.5 together with T=60°C, however there is no dithionite detected in this run. Generally at low sulfite dose together with low current, both final current efficiency and brightness gain are low. The repeatability of dithionite concentration and current efficiency is poor. This is mainly because dithionite is unstable, and slight difference in operating conditions during the experiment can cause a big difference in dithionite concentration.  The results of this factorial analysis are listed in Tables D.20 of Appendix D. Only T has a significant effect on brightness gain, and increases in T increase brightness gain. No significant effect on final current efficiency is found in the variable ranges studied in this experiment.  B. High Sodium Sulfite Dose Electrochemical Brightening at Elevated Temperature with Half Cylinder Cathode  The reaction system studied above can not generate sodium dithionite at most combinations of temperature and pH level, and this could miss the opportunity to achieve high brightness gain (20 % ISO). Since previous experiments in Section 4.4 showed that at high sulfite dose, dithionite can be obtained at all combinations of temperature and pH, a fhrther experiment at the high level of sodium suffite dose was conducted in hope of achieving high brightness gain. A 3 level, 3 factor (I, T and pH) replicated factorial experiment was carried out.  The operating conditions, symbols, experimental variables and results are listed in Tables D.21, D.22 and D.23 of Appendix D. Run average values of the time average dithionite concentration are equivalent to 12 wt% to 96 wt% on od pulp with final current efficiency of 22% to 78%. However, the highest average value of the brightness gain in 67  Chapter 4. Experimental Results and Discussion this experiment is only 8.6 % ISO, and this is lower than the brightness gain of the conventional dithionite brightening at 1 % pulp consistency with 1 wt% dithionite charge on od pulp at 60 °C (10.1 % ISO).  The results of the factorial analysis are listed in Table D.24 of Appendix D. The factors T and I have significant effects on brightness gain, listed in declining sequence, the standard deviation of the brightness gain is 0.4 % ISO, and increases in T and I increase the brightness gain. Generally run average values of the final current efficiency are pretty good, ranging from 45—70%, the standard deviation of the final current efficiency is 6%,  and the factor I has significant effect on final current efficiency. Decreases in I increase final current efficiency. The effects of I on final current efficiency and brightness gain have opposite sign.  68  Chapter 4. Experimental Results and Discussion C. High Sodium Sulfite Dose Electrochemical Brightening at Elevated Temperature with Rectangular Cathode  The half cylinder cathode had 2.4 times area of the rectangular cathode. Previous experiments in Section 4.4 showed that with a bigger cathode area, higher dithionite concentration was obtained, thus the brightening solution composition was different. This difference could have significant impact on brightness gain. Thus in-situ electrochemically generated dithionite brightening with a rectangular cathode at high sodium sulfite dose was carried out to investigate the effect of cathode area on brightening.  The process conditions are the same as those listed in Table D.21, except the cathode is the rectangular one. A 3 level, 3 factor (I, pH and T) factorial design was also conducted, and the experimental variables and results are listed in Table D.25 of Appendix D (with rectangular cathode). Compared to the run average values in Table D.23 (with half cylinder cathode), the corresponding brightness gains in Table D.25 are generally lower, and the final current efficiency and time average dithionite concentration in Table D.25  are  generally higher at high current (4.0 A), but lower at current less than 4.0 A.  Some selected runs with brightness gain exceeding about 8.0 % ISO in Tables D.23 and D.25 are listed in Table 4.7. Table 4.7. Runs with high brightness gain obtained with two different cathode configurations at high level of sodium sulfite dose Run No. BG CE Cathode (%ISO) Configurations (%) Half Cylinder (L3, L3a)* 8.6 25.6 Cathode (L5, L5a) 7.9 59.5 (Li, Lia) 7.9 49.0 M5 Rectangular 8.0 49.3 Ml 8.0 Cathode 47.7 *: average value of the runs in the bracket  69  Chapter 4. Experimental Results and Discussion If the brightness gain is about 8.0 % ISO, higher final current efficiency can be obtained with the half cylinder cathode  In Sections A to C, the highest brightness gain is still lower than the brightness gain of the conventional sodium dithionite brightening at 1 % pulp consistency with 1 wt% sodium dithionite on od pulp at 60 °C (10.1 % ISO). This is probably because dithionite brightening potential decreases in the presence of large amount of (bi)sulfite, as multiple charge dithionite brightening in Section 4.3 reveals.  D. High Sodium Sulfite Dose Electrochemical Brightening at Elevated Temperature with Rectangular Cathode and 2-Propanol  With the rectangular cathode, the maximum brightness gain is still just 8.0 % ISO at 60°C with time average dithionite concentration equivalent to 10 — 95 wt% on od pulp. Obviously, bisulfite and other dithionite brightening products hinder dithionite brightening capability. Figure 4.5 showed that 2-propanol could alleviate the hindrance from bisuffite, thus electrochemically generated dithionite brightening with 2-propanol,  rectangular cathode and high sodium suffite dose was conducted in search of higher brightness gain (> 10 % ISO). A 3 level, 3 factor (I, T and pH) factorial experimental design was carried out.  The process conditions, experimental variables and results are listed in Tables D.26 and D.27 of Appendix D. Table D.27 also includes three runs conducted without 2propanol whose Run Nos. end with  @ and another run conducted without current whose  Run No. ends with #. Under the two conditions compared in this experiment, 2-propanol  has no effect on brightness gain, however, compared to the results in Table D.25, generally the brightness gain in this table are slightly higher, by 2 % ISO. In this study, 70  Chapter 4. Experimental Results and Discussion highest run average brightness gain is 8.5 % ISO at Run Ni, only 0.5 % ISO higher than that in Table D.25, which indicates 2-propanol does not improve the maximum brightness gain. Table D.27 also shows that under condition N2, the brightness gain contributed by dithionite is only about 0.9 % ISO, which is only a small fraction of the total brightness gain.  E. High Sodium Sulfite Dose Electrochemical Brightening at Elevated Temperature with Half Cylinder Cathode and Chromium Nitrate  Chromium sometimes can be a catalyst, but it is also a toxic water pollutant. Thus a brief investigation on electrochemically generated ditbionite brightening at elevated temperature with half cylinder cathode under high sodium sulfite dose and catalyst  —  chromium of about 5 ppm (in pulp suspension) was also conducted. The conditions studied are where the maximum brightness gain are usually obtained, e.g. in Parts B to D. The process conditions, experimental variables and results are listed in Tables D.28 and D.29. Chromium shows no effect on the maximum brightness gain.  F. Yellowness  Yellowness decreases in electrochemically generated dithionite brightening in the same way as in conventional dithionite brightening.  G. Discussion  In pulp brightening with electrochemically generated sodium dithionite, maximum brightness gain is probably controlled by thermodynamic factors (e.g. brightening reaction free energy change), while kinetic factors control the generation of dithionite. 71  Chapter 4. Experimental Results and Discussion  For conventional dithionite brightening, further increase in brightening period and charge beyond a certain level does not increase the maximum brightness. This suggests that the maximum brightness gain is not controlled by the kinetics of the brightening reaction.  Section 2.6, as well as experimental results in Sections 4.3, 4.5 and 4.6 show that temperature basically sets the maximum brightness gain. This is probably because the reaction between chromophores and brightening species requires a certain free energy change. In order for the brightening reaction to proceed, brightening temperature has to be high enough to meet this free energy requirement. After these chromophores are consumed, excess brightening species can not react with other chromophores, thus can not increase the brightness gain.  The real brightening species in conventional dithionite brightening may be the - radical [Wan 1993]. Bisulfite and pH probably have some effect on the formation of 2 SO the SO - radical from dithionite. The S0 2 2 radical may exist in relatively large quantity at weak acidic condition (e.g. pH 4 to 6), thus dithionite has strong brightening power at this pH range. Bisulfite and possibly some dithionite reaction products could have a detrimental effect on the formation of S0 2 radical from dithionite, this may be the reason that in conventional dithionite brightening a charge of dithionite with bisulfite (13 to 131 wt % on od pulp) gave a lower brightness gain than a charge with dithionite alone. For the  same reason increases in sulfite charge from 13 to 131 wt % on od pulp in conventional brightening of pulp using sodium dithionite with sodium (bi)sulfite caused the brightness  gain contributed by dithionite to decrease by 1% ISO. The equilibrium ratio of S02 radical to dithionite may be higher at pH 5.5 than at pH 4.0. This explains the fact that in electrochemical dithionite brightening at pH 5.5 the highest range of brightness gain can 72  Chapter 4. Experimental Results and Discussion be obtained with either low or high dithionite concentration, while at pH 4.0 the highest range of brightness gain can only be obtained at high dithionite concentration. At pH 4.0 - radical for effective brightening. 2 there is probably not enough SO  Sodium dithionite redox potential could also be a factor in brightening. In order to achieve good brightening, the redox potential may have to be in a certain range to attack the corresponding chromophores. The electrode reaction of bisulfite reduction to dithionite is shown as follows: 3 2HS0  +  2H + 2e  —*  S2042  +  21120  VO  =  0.1 V SHE  The redox potential of bisulfite/dithionite couple is -1 2 rso R’T L 2 V’=V°————in nF 2 [H1 ] 3 [HSO  (26)  Increases in dithionite concentration, pH and temperature or decreases in bisulfite concentration decrease dithionite redox potential. If pH increases from 6 to 8, the redox potential could be kept at the same value if dithionite concentration decreases 10000 times. Thus dithionite at pH 6 would have the same brightening power as dithionite at pH 8 with 10000 times less dithionite concentration. So the dithionite redox potential factor is 2 radical may not a factor in the process, although the redox potential of the transient SO be important.  The results in Section 4.4 show that high dithionite concentration is obtained at high suffite dose and high current concentration. This can be explained by the following mechanism. The mechanism of electrosynthesis of sodium dithionite is thought to be as follows [Koithoff and Miller 1941; Oloman 1994]: 2 SO  +  2 2SO  -  e  —*  2 SO  (27)  -  (28)  s -2 4 o 2  s -2 —* S 4 o 2 4 -2 decomposition products 0 2 73  (29)  Chapter 4. Experimental Results and Discussion Thus the net dithionite reaction rate can be expressed as two terms: first, dithionite formation rate on the cathode surface and second, dithionite decomposition rate in the bulk solution.  d[S ] O 2 dt  =  2 [ 1 k ] SO A, —k22 [S Q2] V  (30)  1 k  rate constant of dithionite formation reaction (Reaction (27)), m s’.  2 k  . 1 rate constant of dithionite decomposition reaction, s  A  . 2 cathode surface area, m  Under weak acidic condition, pH 4 —6, a fraction of the (bi)sulfite dose added . If the pH is higher than 6, no SO 2 2 will exist, thus into the system will be present as SO no dithionite will be generated. At pH 4—6, the higher the (bi)suffite dose, the higher the 2 concentration, thus dithionite formation rate will increase. With decreasing pH, a SO , thus 2 higher fraction of the (bi)sufflte added into the system will be present as SO dithionite formation rate will increase. Also increase in the ratio of cathode area to cathode reaction volume Ac/V will favor dithionite accumulation. Increase in temperature will increase k 1 slightly.  (31)  S0 2 [ 1 k j =nF1ICE CE  current efficiency for Reaction (27)  If other conditions are the same, increases in current density (below the mass transport limiting current density for Reaction (27)  )  will increase dithionite formation  rate. However increases in current density will also increase the dithionite concentration  and thus increase the dithionite decomposition rate. There will be an optimum current density for dithionite accumulation.  Increases in temperature or decreases in pH will increase k 2 and thus increase the rate of dithionite decomposition.  74  Chapter 4. Experimental Results and Discussion  The optimum conditions for dithionite electrosynthesis and for brightening of pulp with dithionite are in conflict. Pulp brightening requires high temperature (above 60°C), pH around 5.5, with little or even no (bi)sulfite present in the system. Electrosynthesis of dithionite requires low temperature (ambient temperature or even lower), pH 4—5.5 and high (bi)sufflte dose. The main difficulty in electrochemical brightening of pulp with sodium dithionite generated in-situ is to achieve high time average dithionite concentration (at least 5 g/l) at high temperature (80—90 oC).  75  Chapter 4. Experimental Results and Discussion 4.7 Optimization of Bri!htness Gain in Electrochemical Dithionite Bri2htenin with Half Cylinder Cathode at 2% Consistency  Parts C to E in Section 4.6 revealed that cathode area, addition of 2-propanol and chromium did not significantly increase maximum brightness gain, and Part B in Section 4.6 revealed that increases in current and temperature raised brightness gain, thus sequential simplex optimization was conducted at 2% pulp consistency from initial conditions of T  =  60°C together with I  =  4 A. The process conditions, experimental  variables and results are listed in Tables D.30 and D.31. The brightness gain versus run number of optimization is also plotted in Figure 4.9.  Run Qi to Run Q5 are starting points, four runs of optimization are carried out. However, the responses obtained are at the same level as the best response of the starting points. This indicates that the searching area is already located on the ridge. From four runs of optimization, we can see that among four operating variables, only temperature increases steadily which shows that temperature effect on brightness gain is very significant, current and dose settle down at between 5 —6 A and 50 —60 g, while pH remains 4  —  5. Due to the experimental set up limitation, 83  —  84 0 C is the highest  brightening temperature which can be achieved with good control in this experiment. Further increase in temperature may only increase brightness gain slightly since from 60— 80 0 C (Run Q1 to Run Q5), brightness gain increases only 2.4 % ISO, while from 73.8  —  80.6 0 C (Run Q7 to Run Q8), brightness gain remains unchanged. The highest range of brightness gain (11 —12 % ISO) is only obtained at the highest range of temperature (74 —  83 OC), with time average dithionite concentration equivalent to 7  pulp and very poor final current efficiency (0—13 %).  76  —  10 wt% on od  Chapter 4. Experimental Results and Discussion  14• 13•  /Vv 7.  CD a  6 5, 4, 321—  •  1  ..i•i.i.i,i.i•  2  3  4  5  6  7  8  9  I  10  Run No.  Figure 4.9. Sequential Simplex optimization of electrochemical brightening  77  Chapter 4. Experimental Results and Discussion 4.8 Blank Experiment (without Current  In order to determine the brightness gain contributed by the brightening species generated by current, a set of blank experiments (without current) was conducted. The experiment was carried out in the stainless steel electrochemical reactor. The process conditions, experimental variables and results are listed in Tables D.32 and D.33. The standard deviation of this study (0.4% ISO) is estimated from two pairs of repeated runs:  Runs R3, R3a and Runs R4, R4a. Comparing the brightness gain of the blank experiment (no current) to the highest brightness gain obtained from electrochemical brightening at the same sodium suffite dose, temperature, pH and pulp consistency, the maximum further brightness gain contributed by the brightening species generated by current is only about 4 % ISO. For a brightness gain of 11.4 % ISO, the fraction of the brightness gain contributed from the brightening species generated by current is about 26% of the total brightness gain.  78  Chapter 5. Conclusions and Recommendations Chapter 5  Conclusions and Recommendations  5.1 Conclusions  So far in-situ electrochemically generated dithionite brightening could not achieve as high brightness gain as conventional peroxide brightening. However, many worthwhile observations were made throughout the course of this thesis.  1. Hydrazine sulfate, hydroquinone, hydroxylamine sulfate and sodium hypophosphite were less effective in brightening mechanical pulp than sodium dithionite.  2. With the same dithionite charge, brightness gain by dithionite at 1 % pulp consistency was at the same level as in conventional brightening at 3 to 4% pulp consistency. Beyond 1 wt% sodium dithionite charge, further increases in dithionite charge did not increase brightness gain.  3. In conventional sodium dithionite brightening at 1% pulp consistency and 60°C, addition of dithionite in multiple stages with interstage washing achieved slightly higher brightness gain (about 1.0 % ISO) than multiple addition in a single run with the same total dithionite charge; and multiple addition achieved slightly higher brightness gain (about 1 % ISO) than single addition with the same total dithionite charge. The time and the amount of each charge did not seem to affect the brightness gain in multiple charge dithionite brightening.  79  Chapter 5. Conclusions and Recommendations 4. In dithionite brightening with a large amount of (bi)sulfite, the brightness gain was lower than that with dithionite alone, but slightly higher than that with only sulfite. Sodium bisulfite and other dithionite reaction products seemed to decrease dithionite brightening capability. The addition of 2-propanol and methanol alleviated this negative effect of sodium bisulfite on dithionite brightening by about 1% ISO.  5. In the electrochemical synthesis of sodium dithionite (without pulp) at elevated temperature, high time average dithionite concentration (5 to 10 g/l) was obtained at high current ( 2.3 A) together with high sodium suffite dose ( 25.3 g) with fairly good current efficiency for dithionite generation (35 to 80  %), however, sodium  dithionite concentration was very low (0 to 1 g/l) at low sulfite dose (4.6 g). Cathode area, current and sodium sulfite dose were very significant effects in this process. Increases in these variables increased time average dithionite concentration. Increases in cathode area as well as increases in suffite dose raised the final current efficiency for dithionite generation. Decreases in current raised final current efficiency with the 2 area, but did not have significant effect with cathode of 13000 cathode of 5500 mm 2 area. mm  6. When electrochemical dithionite brightening was carried out at 0.8 % pulp consistency in the following ranges: pH (4.0 to 5.5), sodium sulfite dose (4.6 to 46.0 g), current (0.5 to 4.0 A) and temperature (40 to 60 OC), the highest range of brightness gain was 8 to 9 % ISO with time average dithionite concentration equivalent to a charge of 10 to 103% on od pulp. This highest range of brightness gain was obtained at any pH with a combination of high sulfite dose (46.0 g), high temperature (60 °C) and high current (4 A). This range of brightness gain was also obtained at pH 5.5 with a combination of high sulfite dose (46.0 g), high temperature (60 °C) and low current (0.5 A). Temperature had the most significant effect on brightness gain in this process, 80  Chapter 5. Conclusions and Recommendations increases in temperature increased brightness gain but decreased final current efficiency. Cathode area, addition of 2-propanol and chromium did not increase this highest range of brightness gain in the ranges of operating variable studied.  7. Brightness gain for electrochemical brightening of pulp with sodium dithionite generated in-situ at 2% pulp consistency was optimized. The highest range of brightness gain in this study was 11 to 12 % ISO. This was only obtained at the highest range of temperature (74 to 83 OC), with time average dithionite concentration equivalent to a charge of 7 to 10 % on od pulp and very poor final net current efficiency for dithionite generation (0 to 13 %).  8. Comparing the brightness gain of the blank experiment (no current) to the highest brightness gain obtained from the electrochemical dithionite brightening at the same sodium sulfite dose, temperature, pH and pulp consistency, the maximum further brightness gain contributed by the brightening species generated by current was only about 4 % ISO. For a brightness gain of 11.4 % ISO, the fraction of the brightness gain contributed by the brightening species generated by current was 26 % of the total brightness gain.  9. Yellowness decreased in electrochemically generated dithionite brightening in the same way as in conventional dithionite brightening.  81  Chapter 5. Conclusions and Recommendations 5.2 Recommendations  The following recommendations are suggested for fhture work in the in-situ electrochemically generated dithionite brightening:  1. Investigate brightening suspension composition, such as the concentration of sulfide, thiosulfate, bisuffite and dithionite during the brightening reaction. This may explain why the brightness gain is not additive even though both brightening agents, bisuffite and dithionite are present at very high levels. This will also provide information about  the feasibility of recycling the brightening solution.  2. Further investigate electrochemically generated dithionite brightening at very high temperature (i.e. above 90 °C) with high time average dithionite concentration (i.e. 50 to 100 % on od pulp) in hope that very high brightness gain (20 % ISO) might be achieved. In order to achieve these brightening conditions, a new electrochemical reactor is needed, probably with high ratio of cathode area to cathode chamber volume, water cooled cathode and in a pressurized vessel. This new electrochemical reactor may improve the process performance.  3. Further investigate some chemicals which can suppress the negative effects of some dithionite reaction products on dithionite brightening capability.  4. Further investigate electrochemical dithionite brightening at higher pulp consistency, i.e. 3 -4 % to improve the process economy.  82  Bibliography  Bibliography  Alder, E., “Lignin Chemistry—Past, Present and Future”, Wood Science. Technol. 11, 169-218 (1977). 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(Olin Corporation), “Private Communication”, 1994. Kerekes, R.J., Soszynski, R.M. and Tam Doo, P.A., “The Flocculation of Pulp Fibers”, Transactions of the Eighth Fundamental Research Symposium, “Papermaking Raw Materials”, V. Punton, Ed., Fundamental Research Committee, 1, 265-310 (Oxford, 1985) Kerekes, R.J., “Mechanical Pulping”, Course Notes of CHML471, 1991. Kolthoff I.M. and Miller, C., “The Reduction of Sulfurous Acid (Sulfur Dioxide) at a Dropping Mercury Electrode”, J. Am. Chem. Soc. 63, 28 18-2821 (1941). Kutney, G.W. and Evans, T.D. “Sulfur Dioxide and Its Derivatives: The Forgotten Bleaching Chemicals”, J. of Pulp and Paper Science: Vol. 12. No.2, J44-J49 (March, 19 86). Leask, R.A., and Kocurek, M.J., Editors, “Pulp and Paper Manufacture ( 3rd Ed. ), Vol 2., Mechanical Pulping”, Joint Textbook Committee of the Paper Industry (TAPPI and CPPA), Atlanta, 1987, chapter XIX.  84  Bibliography Leutner, B., et al., “Continuous M.nufacture of Sodium Dithionite Solutions by Cathodic Reduction”, US Patent 4,144,146, 1979 (To BASF). Mcdonough, T.J., “A Survey of Mechanical Pulp Bleaching in Canada”, Pulp & Paper Canada 93:4, T108-1 10 (1992). Meizer, 3., “ Stability of Sodium Dithionite in Aqueous Solutions”, Wochenblatt fur Papierfabrikation, 21, 925-931(1990). A Novel Method for Bleaching Kraft and Suffite Nassar, “Electrochemical Bleaching Puips”, J. Pulp and Paper Science, Vol. 11, No. 1, J11-20 (1985). —  Murphy, T.D., “Design and Analysis of Industrial Experiments”, Chem. Engr., 84(12), T168 182 (1977). -  Oloman, C. (University of British Columbia), “Private Communication”, 1994. Oloman, C., et al, “Electrosynthesis of Sodium Dithionite in a Trickle-bed Reactor”, Can. J. Chem. Eng., 68, 1004-1009 (1990). Oloman, C., “ The Preparation of Dithionites by the Electrolytic Reduction of Sulfur Dioxide in Water”, 3. Electrochem. Soc., Vol.117, No.12, 1604 1609 (1970). -  Oloman, C., and Austin, R., “The Electrolytic Production of Sodium Dithionite for Groundwood Brightening”, Pulp Paper Mag. Can., 70(24): T529-T534 (1969). Pemg, Y.S. “Electrochemically Mediated Oxygen Bleaching of Pulp”, Ph.D. Thesis, Department of Chemical Engineering, University of British Columbia, 1993. Polcin, 3. and Rapson, W.H., “Effects of Bleaching Agents on the Absorption Spectra of Lignin in Groundwood Pulps: Part I and Part II”, Pulp Paper Mag. Can. 72 no. 3: 69-91 (T103-24), (1971). Pinker, R., et al., “Kinetics and Mechanism of the Thermal Decomposition of Sodium Dithionite in Aqueous Solution”, md. Eng., Chem. Fundamentals, 4, 282 288 (1965). -  85  Bibliography  Singh, R.P., “The Bleaching of Pulp”, TAPPI monograph 3rd ed., TAPPI, Atlanta, 1979, Chapter 9. Smook, G.A. “Handbook of Pulp & Paper Terminology”, Angus Wilde Pub., Vancouver, 1990. Smook, G.A., “Handbook for Pulp & Paper Technologists”, 2nd Ed., Angus Wilde Pub., Vancouver, 1992. Spencer, M. S., “Chemistry of Sodium Dithionite. Part I: Kinetics of Decomposition in Aqueous Bisuffite Solutions”, Trans. Faraday Soc., 63, 2510 2515 (1967). -  Todd, R., “The Electrochemical Mediation of Oxygen Delignification of Pulp with a Manganese Polyol Complex”, M.A.Sc. Thesis, University of British Columbia, 1993. Tredway, C.M., “Factors Affecting the Bleaching of (3roundwood with Sodium Hydrosuffite”, Pulp and Paper, 53(3): 73 -75 (Mar., 1979). Walters, F.H.,  Ct  al, “Sequential Simplex Optimization”, CRC, Boca Raton, 41-42 (1991).  Wan, 3. (Queen’s University), “Private Communication”, 1993. Wilk, I.J.,”Economic Advantages of Electrolysis System for Pulp Bleaching”, Tnt Symp Wood and Pulping Chemistry, TAPPI, 135—137 (Raleigh, 1989).  86  Appendices Appendix A Measurement of Pulp Stock Consistency  A sample of about 15 g pulp stock is taken and weighed, and the weight is recorded as wet weight. A handsheet is made according to the procedure set in CPPA standard C.5, dried in an oven at 105°C for 24 hours, then transferred immediately into a desiccator. After the handsheet is cooled, it is weighed again and the weight is recorded as  dry weight. The procedure is repeated with a second sample of pulp stock. The consistency is the dry weight divided by the wet weight. The final consistency is the average of these two measurement.  87  Appendices Appendix B PuJp Specification  Pulp:  Howe Sound Pulp and Paper Ltd. (1) TMP 1, 70% SPF(Spruce, Pine, Fir), 30% Hembal; (2) TMP 2, 55% SPF(Spruce, Pine, Fir), 45% Hembal;  88  Appendices Appendix C Real Surface Area of Cathode Screen  2xD 2x2 A=3.14 x DxL x (M+1) x 2- (M+1) A:  Real surface area (n’.m ) 2  L:  25.4mm(1 inch)  M:  Screen mesh number  D:  Nominal wire diameter (mm)  (1). Real surface area of 25.4mm x 25.4 mm (1” x 1”) plate 0 =2 x 25.42 A  =  1290 mm 2  (2). Real surface area of 25.4mm x 25.4 mm screen 1 of mesh 60, wire diameter 0.1905 mm (0.0075”). 1 A =  =  3.14 x 0.1905 x 25.4 x (60+1) x 2- (60+1)2  1854-270  =  <  0.19052 x 2  2 1584mm  Ratio of real surface area of screen 1 to plate with same superficial surface area 1 /A A 0  =  1584/1290 = 1.228  (3). Real surface area of 25.4mm x 25.4 mm screen 2 of mesh 8, wire diameter 0.63 5mm (0;025”). 2 = 3.14 x 0.635 x 25.4 x (8+1) x 2- (8+1)2 x 0.6352 x 2 A 911.6-65.3  =  846.3 inch 2  Ratio of real surface area of screen 2 to plate with same superficial surface area 2 /A A 0  =  846.3/1290 = 0.656  89  Appendices Appendix D Experimental Results  Table D. 1. Brightness responses versus charge at 1% consistency 4 O S 2 Na  Charge Results  on od Pulp (wt%) BG*  (% ISO)  YL*  1  10.1  4.6  2  10.9  5.1.  4  10.2  5.7  (%)  8 10.1 5.1 Operating conditions: consistency 1%! STPP 0.5% on od pulp! pH . 2 5.0! 60 O/ 60 min.1N Original pulp (TMP 1): B = 49.8 % ISO, Y = 30.5 %. * BG = Brightness Gain, YL = Yellowness Loss.  90  Appendices Table D.2. Multi le charge sodium dithionite brightening at ambient temperature Run No.  Operating Conditions  B1  B2 ,  1% consistency/i wt% dithionite on od pulp /2 wt% STPP on od pulp/pH 5.5 1T20°C/ 1 hr / N 9 1% consistency /2 1% dithionite eveiyl5min/2wt%STPPonod pulp/pH 5.5 T=20°C/ 1 hr / N 9  Original pulp (Th1P 1): B  =  Results BG (% ISO)  YL  4.3  2.0  5.1  2.2  (%)  52.4 % ISO, Y = 29.6 %.  Tnble D.3. Multi le charge sodium dithionite bri htening at elevated temperati1 Run No.  B3 (1)  B4 (2) B5  B6  B7  Operating Conditions 1% consistency/i wt% dithionite on ad pulp/2 wt% STPP on od pulp/pH 5.5 IT=60°C/ 1 hr / N 9 1% consistency/i wt% dithionite on od pulp/2 wt% STPP on ad pulp/pH 5.5 [P60°C/ 1 hr / N 9 1% consistency /2 wt% dithionite onodpulpeveryl5min/2wt% STPP on od pulp/pH 5.5 Ir=60°C/ 1 hr / N 7 twice “1% consistency /1 wt% dithionite on od pulp/2 wt% STPP on od pulp/pH 5.5 1r=60°CJ 30 mm / N ” with 2 interstage washing 1% consistency/i wt% dithionite on od pulp evely 30 min/2 wt% STPP on od pulp/pH 5.5 /60°C/ 1 hr / N 9  Original pulp (TMP 1): B  =  Results BG (% ISO)  YL (%)  8.7  4.2  9.5  4.8  9.9  6.0  10.9  5.3  10.1  5.4  52.4 % ISO, Y = 29.6 %.  91  Appendices Table D.4. Multiple charge sodium dithionite brightening with sodium sulfite at elevated temperature Run No.  Operating Conditions  B8  1% consistency /0 wt% dithionite on od pulp/2 wt% STPP on cxl  Results BG  (% ISO)  YL  4.3  3.3  onodpulpeveryl5min/2wt°h 7.4 STPP on od pulp/pH 5.5 /T=60°C/ 1 hr / N ! 131 wt% 2 sodium sulfite on od pulp 1% consistency /0 wt% dithionite on od pulp/2 wt% STPP on od 2.9 pulp/pH 5.5 /T’60°C/ 1 hr / N 2/ 13.1 wt% sodium sulfite on od  4.9  pulp/pH 5.5 /T=60°C/ 1 hr / N / 2 131 wt% sodium sulfite on od pulp 1% consistency /2 wt% dithiomte  B9  BlO  1.4  pulp  1% consistency /2 wt% dithionite on od pulp every 15 mm /2 wt% 6.9 STPP on od pulp/pH 5.5 1=6O°Ci 1 hr / N 2 / 13.1 wt% sodium sulfite on od pulp Original pulp (TMP 1): B = 52.4 % ISO, Y = 29.6 %. B 11  92  3.1  (%)  Appendices  Table D.5. Multiple charge sodium dithionite brightening with sodium sulfite and suppresser at elevated temperature Run No.  Operating Conditions  B12  1% consistency/i wt% dithionite on od pulp/2 wt% STPP on od pulp/pH 5.5 /T=60°C/ 1 hr / N ? 2  Results BG(% ISO)  YL(%)  -0.9  5.6  8.7  4.4  2.7  1.0  8.0  4.1  13.1 wt% sodium sulfite on od  B13  B 14  B 15  B16  B 17  B 18  pulp/O.3 wt% silver nitrate on od pulp 1% consistency/i wt% dithionite on od pulp/2 wt% STPP on od pulp/pH 5.5 /T=60°C/ 1 hr / N ? 2 20 wt% 2-propanol on od pulp 1% consistency /0 wt% dithionite on od pulp/2 wt% STPP on od pulp/pH 5.5 /T=60°C/ 1 hr / N ! 2 13.1 wt% sodium suffite on od pulp! 20 wt% 2-propanol on od pulp 1% consistency /2 wt% dithionite on od pulp every 15 min/2 wt% STPP on od pulp/pH 5.5 IT=60°C/ 1 hr / N ! 13.1 wt% 2 sodium sulfite on od pulp! 20 wt% 2-propanol on od pulp 1% consistency /0 wt% dithionite on od pulp!2 wt% STPP on od pulp/pH 5.5 /T=60°C/ 1 hr / N 2/  1.2  20 wt% methanol on od pulp 1% consistency/i wt% dithiomte on od pulp!2 wt% STPP on od 7.6 pulp/pH 5.5 /T=60°C! 1 hr / N 2 /13.1 wt% sodium sulfite on od pulp! 20 wt% methanol on od pulp 1% consistency/i wt% dithiomte on od pulp/2 wt% STPP on od 1.7 pulp/pH 5.5 /T’60°C/ 1 hr/N ! 2 13.1 wt% sodium sulfite on od pulp/ 10 wt°h formaldehyde on odpulp  Original pulp (TMP 1): B  =  52.4 % ISO, Y = 29.6 %.  93  0.4  3.5  0.4  Appendices Table D.6. Operating conditions of sodium dithionite synthesis at elevated temperature with rectangular cathode Operating  reaction volume: 500 ml  Conditions  reaction time:  1 hr  nitrogen purge rectangular cathode Anolyte  0.1 to 1.0 M sulfuric acid  Catholyte  2 on od pulp 1 wt% EDTANa  Composition  46.0 g 3 SO or 2 Na 25.3 g 3 SO or 2 Na 4.6 g 3 SO 2 Na  +  44.8 g 4 SO 2 Na  Table D.7. Symbols used in factorial design of sodium dithionite synthesis at elevated temperature with rectangular cathode Variable  Symbol  Level +  Current (A)  I  4.0  SO Dose (g) 2 Na 3  C  46.0  0.5 4.6 Na 3 S 2 O 44.8 4 SO 2 Na  pH  pH  5.5  4.0  Temperature (°C)  T  60  40  94  +  Appendices Table D.8. Experimental variables and results of sodium dithionite synthesis at elevated temperature and with rectangular cathode 2 pH I Time Ave. Final CE* C T *1 Cone. (g/1) (%) 4.0  46.0  5.5  60  6.37  40.9  4.0  46.0  5.5  40  4.94  34.2  4.0  46.0  4.0  60  6.95  38.5  4.0  46.0  4.0  40  7.27  45.6  4.0  4.6  5.5  60  0  0  4.0  4.6  5.5  40  0  0  4.0  4.6  4.0  60  0  0  4.0  4.6  4.0  40  0  0  0.5  46.0  5.5  60  0.83  56.3  0.5  46.0  5.5  40  0.03  7.7  0.5  46.0  4.0  60  0.32  23.0  0.5  46.0  4.0  40  0.09  9.1  0.5  4.6  5.5  60  0  0  0.5  4.6  5.5  40  0  0  0.5  4.6  4.0  60  0  0  0.5  4.6  4.0  40  0  0  2.3  25.3  4.8  50  5.10  63.4  25.3 4.8 50 2.3 5.21 63.4 Time Ave. Cone. = Time Average Sodium Dithionite Concentration. *2 CE = Current Efficiency for Dithionite.  *1  95  Appendices  Run I  II  Table D.9. Comparison of the effect 4 SO on generating 2 ofNa dithionite I T Time Ave. C pH Conc. (g/l) 4.0 46.Og 5.5 60 6.37 SO 2 Na 4.0 46.Og + 5.5 60 6.82 SO 2 Na 3 44.8g 4 SO 2 Na 46.Og 4.0 SO 2 Na 4.0 40 7.28 4.0 + 4.0 46.Og SO 2 Na 3 40 6.91 44.8g SO 2 Na 4  sodium Duff, between Two Runs 7.4%  -5.0%  Table D.10. Factorial analysis of sodium dithionite synthesis at elevated temperature with rectangular cathode  *  Factor  Time Ave. Conc. (g/l)  Final CE (%)  I  3.03  8  C  3.35  32  pH  -0.31  3*  T  0.27  8  IxC  -3.03  -8  IxpH  0.42  5*  IxT  -0.01w  8  CxpH  0.31  CxT  -0.27  pHxT  -0.29  IxCxpH  -0.42  IxCxT  0.01*  -8  IxpHxT  0.15*  ..3*  CxpHxT  0.29  6*  IxCxpHxT  0.15*  3*  -8  S.D. 0.08 0.00 Insignificant effect (confidence level is 95%)  96  Appendices Table D. 11. Operating conditions of sodium dithionite synthesis at elevated temperature with half cylinder cathode Operating  reaction volume: 500 ml  Conditions  reaction time:  1 hr  nitrogen purge half cylinder cathode Anolyte  0.1 to 1.0 M sulifiric acid  Catholyte  1 wt% EDTANa 2  Composition  46.0 g sodium sulfite or 25.3 g sodium sulfite or 4.6 g_sodium_sulfite + 44.8_g_sodium_sulfate  Table D. 12. Symbols used in factorial design of sodium dithionite synthesis at elevated temperature with rectangular cathode Variable  Symbol  Level +  Current (A)  I  4.0  SO Dose (g) 2 Na 3  C  46.0  0.5 4.6 Na 3 S 2 O 44.8 4 SO 2 Na  pH  pH  5.5  4.0  Temperature (°(D)  T  60  40  97  +  Appendices Table D.13. Experimental variables and results of sodium dithionite synthesis at elevated temperature with half cylinder cathode I pH C T Time Ave. Final CE (%) Cone. (gf1)  *  4.0  46.0  5.5  60  9.23  62.4  4.0  46.0  5.5  40  10.42  78.1  4.0  46.0  4.0  60  8.50  48.7  4.0  46.0  4.0  40  9.56  70.3  4.0  4.6  5.5  60  0  0  4.0  4.6  5.5  40  0  0  4.0  4.6  4.0  60  0  0  4.0  4.6  4.0  40  0  0  0.5  46.0  5.5  60  1.02  60.7  0.5  46.0  5.5  40  1.29  78.4  0.5  46.0  4.0  60  0.95  59.4  0.5  46.0  4.0  40  1.29  82.9  0.5  4.6  5.5  60  0.94*  55.0*  0.5  4.6  5.5  40  0.97  54.2  0.5  4.6  4.0  60  0.00  0.6  0.5 40 4.6 4.0 0.06 5.2 Average value of the two repeated runs listed in Part I of Table D. 14.  98  Appendices Table D. 14. Repeated runs of sodium dithionite synthesis at elevated temperature with half cylinderar cathode Time Ave. Cone. (g/l) Final CE (%) Run I C pH T I  II  0.5  4.6  5.5  60  0.96  55.8  0.5  4.6  5.5  60  0.92  54.1  2.3  25.3  4.8  50  6.11  80.6  2.3  25.3  4.8  50  5.98  80.8  Table D.15. Factorial analysis of sodium dithionite synthesis at elevated temDerature with half cylinder cathode Factor  Time Ave. Cone. (g/l)  Final CE  I  3.90  -17.1  C  5.04  53.2  pH  0.44  15.2  T  0.37  -10.3  IxC  -4.39  -11.6  IxpH  0.04*  9.8  IxT  0.20  CxpH  0.02*  10.6  CxT  0.35  9.3  pHxT  0.01*  -2.1  IxCxpH  0.42  16.0  IxpHxT  -0.22  0.01*  IxCxT  0.03*  CxpHxT  -0.01w  IxCxpHxT  0.02*  *  0.8*  0.6 S.D. 0.05 Insignificant effect (confidence level is 95%)  99  (%)  Appendices Table D.16. Operating variables and experimental results of in-situ electrochemically generated dithionite brightening at ambient temperature mi. pH I BG YL Final SO 2 Na Run 3 t * 1 (A) (mm.) (% ISO) Cone. *2 (%) No. Dose (g) Fin. pH (g/l) -  Cia  26.3  5.5-6.1  1.0  60  3.8  2.4  0.73  Cib  26.3  5.5 6.2  1.0  60  4.3  2.8  n.a.  C2  26.3  5.5 5.7  1.0  10  2.7  2.4  0.44*3  C3  26.3  4.5  5.9  1.0  60  4.6  3.8  2.40  C4  26.3  4.5 5.3  1.0  10  3.9  2.4  0.85*4  C5  26.3  3.5 5.0  1.0  10  3.4  2.1  n.a.  C6  26.3  5.5 5.6  0.1  10  3.4  1.6  n.a.  C7a  5.3  5.5 5.9  0.1  60  3.0  1.5  n.a.  C7b  5.3  5.5 6.0  0.1  60  3.2  1.9  0.00  C8  26.3  5.5 5.5  0  60  3.6  2.1  -  -  -  -  -  -  -  -  -  0 0 2.9 5.5 C9 26.3 Original pulp (TMP 1): B=52.4 %ISO, Y=29.6 % * 1 mi. pH Fin. pH = Initial pH Final pH *2 Final Cone = Final Sodium Dithionite Concentration *3 : Estimated from Run Cia, shown in Section 4.5. *4 : Estimated from Run C3, shown in Section 4.5. -  -  100  1.4  Appendices Table D. 17. Operating condions in electrochemical sodium dithionite brightening with half cylinder cathode at low sodium sulfite dose Operating  reaction volume: 500 ml  Conditions  reaction time: current:  1 hr 0.5 A  nitrogen purge half cylinder cathode Anolyte  0.1 to 1.0 M sulfuric acid  Catholyte  0.8% pulp consistency  Composition  2 on od pulp 1 wt% EDTANa 4.6 g sodium sulfite + 45.75 g sodium sulfate  Table D. 18. Symbols used in factorial design of electrochemical sodium dithionite brightening with half cylinder cathode at low sodium sulfite dose Variable  Symbol  Level +  pH  pH  5.5  4.0  Temperature(°C)  T  60  40  101  Appendices Table D. 19. Experimental variables and results in electrochemical sodium dithionite brightening with half cylinder cathode at low sodium sulfite dose Run No pH T BG YL Time Ave. Final CE Conc.(gfl) (%) (% ISO) (%) K5  5.5  60  5.9  4.2  0  0  K6  5.5  40  4.5  3.5  0.22  12.8  K7  4.0  60  5.2  2.9  0  0  K8  4.0  40  3.6  1.4  0  0  K5a  5.5  60  7.7  4.7  0  0  K6a  5.5  40  3.6  2.9  0.06  1.5  K7a  4.0  60  6.0  3.9  0  0  40 3.9 1.8 0 K8a 4.0 Original pulp (TMP 2, description in Appendix B): B=50.6 % ISO, Y=31.6 %.  0  102  Appendices Table D.20. Factorial analysis of electrochemical sodium dithionite brighteniri with half cylinder cathode at low sodium sulfite dose Effect  pH  T  pHxT  S.D  BG(% ISO)  0.8*  2.3  0.5*  0.8  4* 4* 4* CE(%) * Insignificant effect (confidence level is 95%)  4.  Table D.2 1. Operating conditions of electrochemical sodium dithionite brihtenin with half cylinder cathode at high sodium sulfite dose Operating  reaction volume: 500 ml  Condition  reaction time:  1 hr  nitrogen purge half cylinder cathode Anolyte  0.1 to 1.0 M sulfuric acid  Catholyte  0.8% pulp consistency  Composition  2 on od pulp 1 wt% EDTANa 46.0 g sodium sulfite  Table D.22. Symbols used in factorial design of electrochemical dithionite brightening with half cylinder cathode at high sodium suffite dose Variable  Symbol  Level +  Current (A)  I  4.0  0.50  pH  pH  5.5  4.0  temperature (°C)  T  60  40  103  Appendices Table D.23. Experimental variables and results of electrochemical sodium dithionite brightening with half cylinder cathode at high sodium suffite dose I pH BG YL Time Ave. Final CE Run T Conc.(g/l) (%) No. (% ISO) (%) Li  4.0  5.5  60  8.0  6.1  8.21  52.0  L2  4.0  5.5  40  5.4  4.2  5.35  35.6  L3  4.0  4.0  60  8.9  6.3  4.69  22.0  L4  4.0  4.0  40  5.2  4.2  7.98  52.5  L5  0.5  5.5  60  8.2  6.1  0.94  58.7  L6  0.5  5.5  40  4.2  3.4  1.22  74.6  L7  0.5  4.0  60  6.7  5.0  0.97  56.1  L8  0.5  4.0  40  3.7  2.5  1.14  73.3  L9  2.3  4.8  50  5.6  4.4  5.47  70.3  Lia  4.0  5.5  60  7.7  5.6  7.07  45.9  L2a  4.0  5.5  40  6.0  4.2  3.25  25.0  L3a  4.0  4.0  60  8.2  5.8  5.47  29.2  L4a  4.0  4.0  40  4.0  3.1  5.74  36.6  L5a  0.5  5.5  60  7.5  5.6  1.02  60.2  L6a  0.5  5.5  40  4.3  3.2  1.21  78.4  L7a  0.5  4.0  60  6.6  4.7  0.86  53.1  L8a  0.5  4.0  40  4.2  2.5  0.97  66.2  5.08  63.4  4.8 50 5.7 4.4 L9a 2.3 Original pulp (TMP 2): B=50.6 % ISO, Y=3 1.6 %.  104  Appendices Table D.24. Factorial analysis of electrochemical sodium dithionite brightening with half cylinder cathode at high sodium sulfite dose Effect  I  pH  T  IxpH IxT  pHxT  IxpHxT  S.D  BG(%ISO)  1.0  0.5*  3.1  0.3*  0.2*  0.7*  0.4  10.*  6.  0.1*  5* Final CE (%) -28. 1. Insignificant effect (confidence level is 95%)  Table D.25. Experimental variables and results of electrochemical sodium dithionite brightening with rectangualr cathode at high sodium sulfite dose Run I pH T BG YL Time Ave. Final CE No. Conc. (g/l) (%) (% ISO) (%) Ml  4.0  5.5  60  8.0  6.4  7.52  47.7  M2  4.0  5.5  40  3.3  3.6  7.12  48.6  M3  4.0  4.0  60  5.8  5.1  6.23  32.3  M4  4.0  4.0  40  1.6  1.9  6.76  45.3  M5  0.5  5.5  60  8.0  6.3  0.78  49.3  M6  0.5  5.5  40  7.3  5.0  1.05  71.8  M7  0.5  4.0  60  5.1  4.6  0.80  49.3  M8  0.5  4.0  40  4.2  3.6  0.97  63.3  4.5 3.9 3.34 51.8% ISO, Y= 29.6%.  40.3  M9 4.8 50 2.3 Original pulp (TMP 2): B  =  105  Appendices Table D.26. Operating conditions in electrochemical sodium dithionite brightening with 2-propanol Operating  reaction volume: 500 ml  Conditions  reaction time:  1 hr  nitrogen purge rectangular cathode Anolyte  0.1 to 1.0 M sulfuric acid  Catholyte  0.8% pulp consistency  Composition  1 wt% EDTANa 2 on od pulp 46.0 g sodium sulfite 2.5 wt% 2-propanol on od pulp  106  Appendices Table. D.27 Experimental variables and results of electrochemical sodium dithionite brightening with 2-propanol Run I BO pH T YL Time Ave. Final CE No. (% ISO) (%) Cone. (g/l) (%) Ni 8.6 6.5 48.3 7.59 4.0 5.5 60 Nia 8.3 6.5 3.28 22.0 N2 4.9 4.4 4.87 34.0 4.0 5.5 40 N2a 5.0 4.4 4.28 30.2 N2b# 4.1 3.5 N3 7.3 5.6 6.44 35.7 4.0 4.0 60 N3a 9.5 6.2 3.70 23.4 6.6 9.1 31.4 6.22 N3b@ 7.7 5.1 28.1 4.79 N3c@ N4 4.3 5.1 3.53 25.4 4.0 4.0 40 N5 7.2 6.5 0.84 50.4 0.5 60 5.5 N5a 8.9 6.9 0.91 57.3 7.9 6.7 0.88 57.7 N5b@ N6 6.0 4.8 0.77 57.0 0.5 5.5 40 N6a 5.4 1.09 73.3 N7 6.8 5.4 0.73 43.3 4.0 60 0.5 N8 3.9 3.1 56.5 0.79 0.5 4.0 40 N9 5.9 4.6 5.18 64.9 2.3 48 50 N9a 6.1 4.4 3.19 37.8 Origina’ pulp (TMP 2): B = 52.6 % ISO, Y = 29.9 % ISO. a., b, c : Repeated runs except for those noted by @ and #. @: Without 2-propanol. #: Without current. —  —  107  Appendices Table D.28. Operating conditions in electrochemical sodium dithionite brightening with chromium Operating  reaction volume: 500 ml  conditions  reaction time:  1 hr  current:  4.0 A  temperature:  60 °C  nitrogen purge rectangular cathode Anolyte  0.1 to 1.0 M sulfhric acid  Catholyte  0.8 % pulp consistency  Composition  1 wt% EDTANa 2 on od pulp 46.0 g sodium sulfite 0.5 wt% chromium nitrate nonahydrate on od pulp  Table D.29. Experimental variables and results of electrochemical sodium dithionite brightening with chromium pH Time Ave. Final CE Run No. BG YL Cone. (g/l) (%) (% ISO) (%) P1  4.0  5.82  28.8  7.5  6.7  P2  5.5  8.88  61.3  8.5  6.1  *3  5.5  8.91  62.7  7.5  5.6  *AveL1,Lla  5.5  7.64  49.0  7.9  5.8  8.6  6.0  *AveL3,L3a 4.0 5.08 25.6 Original pulp (TMP 2): B=50.6 % ISO, Y=31.6 %. * Without chromium  108  Appendices Table D.30. Experimental conditions of seciuential simplex optimization Operating  reaction volume: 500 ml  Conditions  reaction time:  1 hr  nitrogen purge half cylinder cathode Anolyte  0.1 to 1.0 M sulfuric acid  Catholyte  2.0 wt% pulp consistency  Composition  1 wt% EDTANa 2 on od pulp SO as in Table D.3 1 2 Na 1  Table D.3 1. Experimental variables and results of sequential simplex optimization T Time Ave. Final SO pH 2 Na 3 Run I BG YL Dose (g) (°C) Conc. (g/l) CE (%) (% ISO) (%) No. (A) Qi  4.0  46.0  5.5  60  4.17  29.1  9.0  6.3  Q2  0.5  46.0  5.5  60  0.80  49.3  7.6  5.8  Q2a  0.5  46.0  5.5  60  1.08  65.7  7.3  5.8  Q3  4.0  23.0*  5.5  60  1.56  12.0  8.7  6.2  Q4  4.0  46.0  4.0  60  1.98  15.3  9.6  6.6  Q5  4.0  46.0  5.5  80  1.64  12.6  11.4  8.0  Q6  5.8  37.4  4.9  67.5  3.78  12.6  9.8  6.9  Q7  5.0  64.7  4.5  73.8  2.03  12.5  11.3  7.5  Q8  5.4  51.0  3.9  80.6  1.43  0.00  11.2  8.2  1.40  12.0  9.1  Q9  51.6 5.1 83.3 1.30 5.7 Original pulp (TMP 2): B=50.6 % ISO, Y=31.6 %. * With 24.9 g sodium sulfate  109  Appendices Table D.32. Operating conditions in blank experiments Operating  reaction volume: 500 ml  Conditions  reaction time:  1 hr  nitrogen purge no current Solution in Anode  a weak sulfuric acid solution with pH of 4.0— 5.5  Chamber Brightening  0.8% to 2.0% pulp consistency  Suspension  1 wt% EDTANa 2 on od pulp  Composition  PH 5.5 Na S 2 O as in Table D.33  R2  Table D13. Experimental results of blank experiments BG(MAX)*l SO 2 Na 3 T ConsistBG YL Dose (g) (°C) ency (%) (% ISO) (%) (% ISO) 6.8 4.6 40 0.8 3.0 1.4 2 (KS K5a)* 4.1 4.6 60 0.8 3.3 1.6 2 (K6, K6a)*  R3  46.0  40  0.8  4.0  2.9  7.3  R3a  46.0  40  0.8  4.6  3.4  (M6)  R4  46.0  60  0.8  6.5  5.4  8.0  R4a  46.0  60  0.8  5.8  4.8  (Ml, orM5)  Run No. Ri  80 46.0 2.0 8.4 R5 7.2 11.4 (Q5) OiiginaI pulp (TMP 2): B50.6 % ISO, Y31.6 %. * 1 BG(MAX) = highest BG in electrochemical brightening at same consistency, temperature, sodium suffite dose and pH *2: Average value of the runs in the bracket.  110  Appendices Appendix E Sample Calculations  Calculations of Time Avera2e Dithionite Concentration and Final Current Efficiency  Example, Run L7:  Operating Variables: Sodium sulfite dose:  46.0 g  Current:  0.5 A  pH:  4.0  Temperature:  60 °C  Brightening period:  1 hr  Molecular weight of sodium dithionite: 174.1 glmol.  At 0, 15, 30, 45 and 60 minutes, the corresponding dithionite concentrations are 0, 0.493, 1.010, 1.454 and 1.823 g/l.  The time average ditbionite concentration is (0 g/l +2 x (0.493 g/l + 1.010 g/l + 1.454 g/l) + 1.823 g/l)/(2 x 4) = 0.97 g/l  The final dithionite current efficiency is calculated using Equation (2) and (3) CE = (2 x 96485 C/mol xO.5 1 x (1.823 g/l / 174.1 g/mol) 1(0.5 A x 3600 sec) = 56.1%  111  Glossary Glossary  Auxochrome: A saturated group which, when attached to a chromophore, alters both the wavelength and the intensity of the absorption maximum (e.g., OH, NH , OMe, Cl). 2 : Brightness. : Brightness gain, which is the brightness difference between brightened pulp and original pulp. Carbonyl: C0 functional group. : Current efficiency. Charge: Amount of chemical added to a batch brightening process, expressed as a weight percentage of oven dried pulp in the process. Chromophore: A covalently unsaturated group responsible for electronic absorption (e.g., C=C, C=O). Conjugation: Two functional groups are close enough within a molecule that one functional group affects the properties of the other. This effect is conjugation. Consistency: A measure of the fibrous materials in pulp suspension. It is expressed as a weight percentage of the total mass. Critical Current Density: The current density corresponding to the maximum yield. CTMP: Chemithermomechanical pulp. Defibration: Separation of wood (or other plant material) into fibers or fiber bundles. Dose: Amount of chemical added to a batch brightening process, expressed as weight. : Degree of polymerization. Handsheet: A single sheet of pulp prepared in the laboratory by draining water from a pulp suspension on a screen-covered sheet mould. It is used in testing the properties of pulp. Hardwoods: Generally, wood produced by a broadleaf tree and containing pores. The term has no reference to the actual hardness of the tree. H.C.: High consistency.  112  Glossary Luminance: Reflectivity of a paper sample measured in green light (wave length 557 nm). Since the eye is most sensitive to green light, this measurement corresponds approximately to how bright a given paper sample appears to average observer. (As opposed to brightness which is a measure of bleaching effect). Mass Transport Limiting Current Density: The current density under pure mass transport control. It is the maximum Faradaic current density which can be supported by the reactant. M.C.: Medium consistency. 24: Oven dried. This is the moisture-free conditions of pulp or paper in the pulp and paper industry. it is determined by drying a known sample to a constant weight in a completely dry atmosphere at a temperature of 100 105 0 C. -  LP: Standard deviation. Softwoods: Generally, wood produced by coniferous tree, i.e., nonporous wood. The term has no reference to the actual hardness of the wood. TMP: Thermomechanical pulp.  I: Yellowness. i: Yellowness loss.  113  

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